Decorative item and process for producing the same

ABSTRACT

A decorative item and method for making same, such as an exterior part of a timepiece, wristwatch band and the like, includes a basis material of stainless steel having a carburized hardened later thereon in which carbon is diffused so as to form a solid solution in which crystalline chromium carbide is not formed and at least one hard coating is deposited on the carburized hardened layer of basis material, wherein the hard coating has a surface hardness greater than that of the carburized layer.

TECHNICAL FIELD

The present invention relates to a decorative item (including parts) anda process for producing the same. More particularly, the presentinvention primarily relates to a decorative item, such as an exteriorpart of a timepiece, in which use is made of a base material having ahardened layer, for example, a carburized (cemented) layer extendingfrom a surface thereof to an arbitrary depth wherein a solute atom isdiffused so as to form a solid solution, and relates to a process forproducing the decorative item. Further, the present invention isconcerned with an exterior part of a timepiece constituted by acarburized stainless steel, especially an exterior part of a timepiece,such as a wristwatch band, bezel, casing, back lid or dial, constitutedby a gas carburized austenitic stainless steel, and is concerned with aprocess for producing the same. Still further, the present invention isconcerned with an exterior part of a timepiece having a smooth orspecular surface free of what is known as “orange peel” and with aprocess for producing the same.

BACKGROUND OF THE INVENTION

In a decorative item, for example, an exterior part (member) of atimepiece, such as a wristwatch band, bezel, casing, back lid, buckle ordial, use is made of stainless steel, titanium or a titanium alloy. Inparticular, austenitic stainless steel which is excellent in corrosionresistance and ornamental capacity is widely employed as the stainlesssteel.

For example, a plate of austenitic stainless steel SUS 316 or SUS 304 issubjected to cold forging. The forged plate is arbitrarily cut ordrilled (punched) and finished into the shape of wristwatch band piece.The thus obtained band pieces are connected to each other to obtain acompleted wristwatch band.

However, the austenitic stainless steel has a drawback in that itsspecular surface is easily scratched to cause the appearance of thewristwatch band, bezel, casing, back lid, dial or other exterior part ofa timepiece constituted by the austenitic stainless steel per se toeasily deteriorate.

The technology of carburizing the surface of stainless steel such asaustenitic stainless steel to harden the stainless steel surface is nowbeing studied in order to resolve the above drawback. However, thecarburized stainless steel surface suffers from generation of a strainin the crystal lattice of stainless steel because of the penetration ofcarbon atoms to become a rough surface. Thus, also, the technology offurther polishing the carburized stainless steel surface into a specularsurface is being investigated.

For example, in Japanese Patent Laid-open Publication No.54(1979)-86441, it is described that a specular surface can be easilyobtained by subjecting fine precision parts, such as a gear, a springand a shaft, constituted by a low carbon steel, a low alloy casehardening steel or the like, although there is no description ofaustenitic stainless steel, to pack carburizing (solid carburizing) at900° C. and thereafter subjecting the surface of such parts to barrelpolishing.

However, when a metal having a high content of chromium, such asaustenitic stainless steel, is carburized at high temperatures such as700° C. or above, chromium carbide is precipitated in a surface portionof stainless steel. As a result, the chromium content of stainless steelper se is reduced to cause the corrosion resistance of the stainlesssteel to extremely deteriorate. Further, the chromium carbide becomesbulky, thereby posing such a problem that the carburized region ofstainless steel cannot have high hardness.

The method of carburizing austenitic stainless steel at low temperaturessuch as less than 700° C. for the purpose of avoiding the aboveprecipitation of chromium carbide can be contemplated. However, when thecarburization is conducted at such low temperatures, a passive filmwhich hinders the penetration of carbon atoms is formed on the surfaceof stainless steel to thereby disenable hardening of the stainless steelsurface.

In recent years, the technology of hardening a surface of austeniticstainless steel while maintaining the excellent corrosion resistance ofthe austenitic stainless steel is being investigated. For example, inthe technology disclosed in Japanese Patent Laid-open Publication Nos.9(1997)-71854, 9(1997)-268364 and 9(1997)-302456, austenitic stainlesssteel is fluorinated in a fluorogas atmosphere at low temperatures suchas 300 to 500° C. to convert the above passive film to a fluorinatedfilm through which carbon atoms can be easily penetrated. Thefluorinated austenitic stainless steel is sequentially subjected to gascarburizing in a carburizing gas atmosphere at low temperatures such as400 to 500° C. and to pickling or mechanical polishing (for example,soft blasting, barrel polishing or buffing).

In the thus-obtained decorative item, for example, a wristwatch band, ahardened layer, i.e., carburized layer extending from a surface ofaustenitic stainless steel as a base material to a depth of 5 to 50 μmis formed while maintaining the corrosion resistance thereof. Therefore,not only does the base material exhibit a beautiful specular surface butalso the specular surface has a Vickers hardness (HV) as high as 500 to700, which cannot be attained by stainless steel provided with nosurface hardening treatment.

The decorative item (including personal ornament) constituted by theaustenitic stainless steel having its surface hardened is resistant toscratching, so that there is an advantage such that the beauty thereofcan be maintained for a prolonged period of time.

However, even if the decorative item whose base material is constitutedby the above stainless steel having its surface hardened sometimessuffers from scratching when a sharp intense external force is appliedthereto.

Therefore, there is a demand for the development of a decorative itemwhose base material is constituted by stainless steel having a highersurface hardness, i.e., greater scratch resistance than that of theconventional decorative item, and also for the development of a processfor producing such a decorative item.

On the other hand, with respect to the decorative item such as anexterior part of wristwatch or a bracelet, it, as a personal ornament,must have the same ornamental value as those of other decorative items.Accordingly, the personal ornament at its surface is often furnishedwith an ornamental coating. For example, a gold alloy coating formed bywet plating is widely employed as such an ornamental coating.

However, the gold alloy coating is soft and easily scratched.Accordingly, even if the hardened base material surface of a personalornament is covered with the soft gold alloy coating, the gold alloycoating would be scratched, spoiling its beauty as a decorative item.This personal ornament has a drawback in that the above advantage ofhardening of the base material surface cannot be utilized.

Therefore, there is a demand for the development of a decorative itemwhose surface hardness as measured from the surface of gold alloycoating is large even if the gold alloy coating per se is soft, that is,a decorative item which is excellent in scratch resistance, and for thedevelopment of a process for producing the same.

In the technology described in Japanese Patent Laid-open PublicationNos. 9(1997)-71854, 9(1997)268364 and 9(1997)-302456, austeniticstainless steel is carburized at low temperature, so that precipitationand bulking of chromium carbide in stainless steel would not occur.However, a layer wherein, mainly, Fe and C in stainless steel aresimultaneously present, possibly “mill scale (roll scale, black scale)”containing an iron oxide such as Fe₂O₃, is formed on an outermostsurface of carburized layer. In the technology described in the aboveliterature, the mill scale is removed by pickling or mechanicalpolishing.

However, with respect to the exterior part of a timepiece constituted bystainless steel which has been gas carburized at low temperature asmentioned above, completely removing the mill scale formed on thesurface thereof so as to render the exterior part of the surfacespecular cannot be accomplished only by performing mechanical polishingsuch as barrel polishing or buffing. The reason is that most timepieceexterior parts have a complex configuration because of the attainment ofornamental beauty with the result that there are places which cannot bepolished, such as the inside wall of holes and inside the wall andbottom of recessed portions. Further, with respect to timepiece exteriorparts comprising a plurality of parts connected to each other, it isdifficult to polish part interfaces. For example, with respect to awristwatch band comprising a multiplicity of band pieces connected toeach other by means of connecting parts, the smaller the interstice ofmutually neighboring band pieces, the more difficult the polishingthereof.

Moreover, the surface of timepiece exterior parts cannot also berendered specular only by pickling. In the pickling described in theabove literature, iron contained in the mill scale is leached with astrong acid solution to remove the mill scale from the surface oftimepiece exterior parts. However, iron is also contained in stainlesssteel per se, so that the surface of carburized layer is corroded by thestrong acid solution. As a result, the surface of carburized layer afterpickling is roughened and cannot be specular.

Furthermore, finishing to be effected on the surface of timepieceexterior parts constituted by stainless steel is not limited to specularfinishing. For realizing an ornamental beauty, various mechanicalfinishings are required. For example, hairline finishing wherein amultiplicity of mutually parallel nicks are engraved or honing wherein amultiplicity of fine recesses are engraved is required.

However, the carburized stainless steel has a drawback in that itssurface is so hard that it is difficult to effect the above mechanicalfinishing thereon.

Therefore, there is a demand for the development of an exterior part ofa timepiece constituted by stainless steel such as austenitic stainlesssteel which is excellent in scratch resistance and has a specularsurface without detriment to the inherent excellent corrosion resistanceof stainless steel; the development of an exterior part of the timepiececonstituted by stainless steel such as austenitic stainless steel whichis excellent in scratch resistance and has its surface provided withmechanical finishing such as hairline finishing or honing withoutdetriment to the inherent excellent corrosion resistance of stainlesssteel; and the development of a process for producing such timepieceexterior parts.

The timepiece exterior parts, for example, band pieces gas carburized atlow temperatures such as 400 to 500° C. as mentioned above are notfurnished with a beautiful specular surface required for exteriorornamentation of timepieces, and the surface thereof is observed as“orange peel” having fine unevennesses, despite the implementation ofpolishing.

The reason is that, by the gas carburization, a greater amount of carbonis diffused within the metal crystal grains of the stainless steelsurface than in the metal crystal grain boundaries. That is, when carbonpenetrates the metal crystal grains, the metal crystal grains becomebulky and swell outward with the result that a thickness differenceoccurs between the crystal grains and the crystal grain boundaries. Whenviewed from the surface of the stainless steel, the crystal grains arehigher than the crystal grain boundaries.

The above height difference between the crystal grains and the crystalgrain boundaries cannot be eliminated despite the implementation of asequence of treatments after the gas carburization, including picklingand mechanical polishing. As a result, the crystal grains are likely tobe recognized as being lifted from the surface of stainless steel, and amultiplicity of lifted crystal grains are observed as fine unevennessesof stainless steel surface, i.e., “orange peel”.

This “orange peel” is a phenomenon which commonly occurs when not onlystainless steel but also titanium, a titanium alloy or other metals foruse in exterior ornamentation of timepieces are subjected to surfacehardening, for example, carburizing at a temperature which is close tothe recrystallization temperature of the metal or below. In particular,the orange peel is a phenomenon which occurs when surface hardening isperformed at below a temperature slightly over the recrystallizationtemperature of the metal.

Further, this “orange peel” is not limited to carburization whereincarbon is used as a solute atom, and is a phenomenon which commonlyoccurs when surface hardening is performed with the use of nitrogen oroxygen as a solute atom at temperature which is close to therecrystallization temperature of the employed metal or below.

Accordingly, there is a demand for the development of an exterior partof a timepiece with an excellent appearance, constituted by a metalwhich has a smooth or specular surface free of “orange peel” even if themetal is subjected to surface hardening at a temperature which is closeto the recrystallization temperature of the metal or below; and for thedevelopment of a process for producing such an exterior part of thetimepiece.

It is an object of the present invention to solve the above problems ofthe prior art and to provide a decorative item comprising a basematerial having a hardened layer, for example, a carburized layerextending from a surface thereof to an arbitrary depth, the basematerial surface having a higher surface hardness, i.e., greater scratchresistance than that of the conventional decorative item, especially anexterior part of a timepiece with such a characteristic.

It is another object of the present invention to provide a decorativeitem comprising the above base material with a hardened layer, thedecorative item having a surface furnished with a golden color or othervarious tones without any lowering of surface hardness, i.e., withoutdetriment to the scratch resistance thereof, especially an exterior partof the timepiece with such a characteristic.

It is an additional object of the present invention to provide processesfor producing the above decorative items.

Further objects of the present invention are to provide an exterior partof a timepiece constituted by stainless steel such as austeniticstainless steel which is excellent in scratch resistance and has aspecular surface without detriment to the inherent excellent corrosionresistance of stainless steel; to provide an exterior part of atimepiece constituted by stainless steel such as austenitic stainlesssteel which is excellent in scratch resistance and has its surfaceprovided with mechanical finishing such as hairline finishing or honingwithout detriment to the inherent excellent corrosion resistance ofstainless steel; and to provide a process for producing such timepieceexterior parts.

It is a still further object of the present invention to provide anexterior part of a timepiece with an excellent appearance, constitutedby a metal which has a smooth or specular surface free of “orange peel”even if the metal is subjected to surface hardening at a temperaturewhich is close to the recrystallization temperature of the metal orbelow; and to provide a process for producing such an exterior part of atimepiece.

SUMMARY OF THE INVENTION

The decorative item of the present invention comprises:

-   -   a base material having a hardened layer extending from a surface        thereof to an arbitrary depth wherein a solute atom is diffused        so as to form a solid solution; and    -   at least one hard coating disposed on a surface of the hardened        layer of the base material.

The solute atom is generally at least one atom selected from the groupconsisting of carbon, nitrogen and oxygen atoms.

The base material is pr4eferably constituted of stainless steel,titanium or a titanium alloy.

The hard coating and the base material at its surface may exhibitrespective tones which are different from each other.

The hard coating preferably has a surface hardness greater than that ofthe base material.

It is preferred that the hard coating be constituted of a nitride,carbide, oxide, nitrido-carbide or nitrido-carbido-oxide of an elementbelonging to Group 4a, 5a or 6a of the periodic table.

The hard coating is preferably a hard coating of carbon.

An intermediate layer may be disposed between the hard coating of carbonand a surface of the hardened layer of the base material.

It is preferred that the intermediate layer comprise a lower layer of Tior Cr disposed on the hardened layer surface of the base material and anupper layer of Si or Ge disposed on a surface of the lower layer.

In the decorative item of the present invention, at least two hardcoatings may be formed on the hardened layer surface of the basis basematerial, or at least two hard coatings may be laminated on the hardenedlayer surface of the base material.

Further, in the decorative item of the present invention, the hardcoating may be disposed on a portion of the hardened layer surface ofthe base material.

The decorative item of the present invention-may further comprise a goldalloy coating disposed on a surface of the hard coating.

It is preferred that the gold alloy coating be constituted of an alloyof gold and at least one metal selected from the group consisting of Al,Si, V, Cr, Ti, Fe, Co, Ni, Cu, Zn, Ge, Y, Zr, Nb, Mo, Ru, Rh, Pd, Ag,Cd, In, Sn, Hf, Ta, W, Ir and Pt.

The decorative item is, for example, an exterior part of the timepiece.

The process for producing a decorative item according to the presentinvention comprises the steps of:

-   -   providing a base material of stainless steel having a hardened        layer extending from a surface thereof to an arbitrary depth        wherein a solute atom is diffused so as to form a solid        solution; and    -   forming at least one hard coating on a surface of the hardened        layer of the base material.

This process enables obtaining the above decorative item of the presentinvention, for example, an exterior part of timepiece such as awristwatch band.

One form of an exterior part of a timepiece according to the presentinvention comprises a stainless steel having at its surface a carburizedlayer wherein carbon is diffused therein so as to form a solid solution(namely, the exterior part of the timepiece comprising a base materialof stainless steel provided at its surface with a carburized layer),

-   -   wherein the carburized layer has a polished surface whose        Vickers hardness (HV) is 500 or more.

Preferably, the polished surface is specular.

Another form of exterior part of timepiece according to the presentinvention comprises a stainless steel having at its surface a carburizedlayer wherein carbon is diffused therein so as to form a solid solution,

-   -   wherein the carburized layer has a machined surface.

It is preferred that the machine surface have a Vickers hardness (HV) of500 or more. This exterior part of the timepiece can be produced bymachining a surface of an exterior part of the timepiece and thereaftercarburizing the machined surface.

One form of a wristwatch band of the present invention comprises aplurality of band pieces of stainless steel connected to each other,

-   -   each of the band pieces having at its surface a carburized layer        wherein carbon is diffused so as to form a solid solution,    -   wherein the carburized layer has a polished surface whose        Vickers hardness (HV) is 500 or more.

Preferably, the polished surface is specular.

Another form of wristwatch band of the present invention comprises aplurality of band pieces of stainless steel connected to each other,

-   -   each of the band pieces having at its surface a carburized layer        wherein carbon is diffused so as to form a solid solution,    -   wherein the carburized layer has a machined surface.

In these wristwatch bands, the band pieces may be connected to eachother by means of connecting parts of stainless steel, each of theconnecting parts having, as at least a portion of its surface, acarburized layer wherein carbon is diffused so as to form a solidsolution.

In the present invention, it is preferred that the wristwatch band beone produced by connecting the band pieces to each other by means ofconnecting parts, carburizing the band pieces and the connecting parts,and thereafter polishing surfaces of the band pieces.

The wristwatch bands of the present invention may further compriseconnecting parts having no carburized layer.

One mode of process for producing a wristwatch band according to thepresent invention comprises the steps of:

-   -   connecting a plurality of band pieces of stainless steel to each        other by means of a plurality of connecting parts of stainless        steel;    -   fluorinating the band pieces and the connecting parts in a        fluorogas atmosphere at 400 to 500° C.;    -   gas carburizing the fluorinated band pieces and connecting parts        in a carburizing gas atmosphere containing carbon monoxide at        400 to 500° C.;    -   pickling the carburized band pieces and connecting parts,        followed by rinsing; and    -   subjecting surfaces of the band pieces to barrel polishing.

A wristwatch band having machined surfaces can be obtained by machiningsurfaces of the band pieces connected by means of the connecting partsprior to the fluorination.

Another mode of process for producing a wristwatch band according to thepresent invention comprises the steps of:

-   -   fluorinating a plurality of band pieces of stainless steel and a        plurality of connecting parts of stainless steel in a fluorogas        atmosphere at 250 to 600° C.;    -   gas carburizing the fluorinated band pieces and connecting parts        in a carburizing gas atmosphere containing carbon monoxide at        400 to 500° C.;    -   pickling the carburized band pieces and connecting parts,        followed by rinsing;    -   subjecting surfaces of the band pieces to barrel polishing; and    -   connecting the band pieces by means of the connecting parts.

A wristwatch band having machined surfaces can be obtained by machiningsurfaces of the plurality of band pieces prior to the fluorination.

The process for producing an exterior part of a timepiece other than awristwatch band according to the present invention comprises the stepsof:

-   -   connecting a plurality of pieces of stainless steel to each        other by means of a plurality of connecting parts of stainless        steel to obtain a base material for a timepiece exterior part of        stainless steel other than a wristwatch band;    -   fluorinating the base material in a fluorogas atmosphere at 250        to 600° C.;    -   gas carburizing the fluorinated base material in a carburizing        gas atmosphere containing carbon monoxide at 400 to 500° C.;    -   pickling the carburized base material, followed by rinsing; and    -   subjecting surfaces of the base material to barrel polishing.

An exterior part of a timepiece other than a wristwatch band havingmachined surfaces can be obtained by machining surfaces of the basematerial prior to the fluorination.

It is preferred that the wristwatch band of the present invention be oneobtained by the above process for producing a wristwatch band accordingto the present invention.

It is also preferred that the exterior part of the timepiece other thanthe wristwatch band according to the present invention be one obtainedby the above process for producing an exterior part of the timepieceother than the wristwatch band according to the present invention.

Austenitic stainless steel is preferably employed as the stainless steelfor use in the present invention.

A further form of an exterior part of a timepiece according to thepresent invention comprises a metal,

-   -   this metal having at its surface a deformed layer containing a        fibrous structure wherein metal crystal grains are deformed so        as to be fibrous, at least the deformed layer having a hardened        layer wherein a solute atom is diffused so as to form a solid        solution.

The above deformed layer is generally formed by application of aphysical external force to at least the surface of the metal. In thepresent invention, it is preferred that the deformed layer be formed byapplication to the metal surface of a physical external force capable ofdrawing the metal surface substantially unidirectionally.

This deformed layer preferably extends from the metal surface to a depthof 2 to 100 μm.

The above hardened layer preferably extends from a surface of thedeformed layer to a depth of 5 to 50 μm.

The above solute atom may be at least one atom selected from the groupconsisting of carbon, nitrogen and oxygen atoms.

The hardened layer preferably has a specular surface whose Vickershardness (HV) is 500 or greater.

The process for producing an exterior part of a timepiece constituted ofstainless steel according to the present invention comprises the stepsof:

-   -   applying a physical external force to a surface of stainless        steel so as for at least the stainless steel surface to have a        deformed layer containing a fibrous structure wherein metal        crystal grains are deformed so as to be fibrous; and    -   dissolving a solute atom in a surface of the deformed layer so        as to form a solid solution therein, thereby effecting such a        hardening that a hardened layer is formed.

It is preferred that the above deformed layer be formed by applicationto the stainless steel surface of a physical external force capable ofdrawing the stainless steel surface substantially unidirectionally.

This deformed layer may be formed by subjecting the stainless steelsurface to at least one of polishing and grinding operations whereby aphysical external force capable of drawing the stainless steel surfacesubstantially unidirectionally is applied to, the stainless steelsurface.

In particular, the deformed layer may be formed by subjecting thestainless steel surface to at least one of cutting and grindingoperations to form a face of desired shape, and

-   -   polishing the face of desired shape to form the deformed layer.

Alternatively, the stainless steel surface may be subjected to agrinding operation to form not only a face of desired shape but also thedeformed layer.

The face of desired shape may be substantially flat, or may be curved.

It is preferred that the deformed layer be so formed as to extend fromthe stainless steel surface to a depth of 2 to 100 μm.

The hardened layer is preferably so formed as to extend from a surfaceof the deformed layer to a depth of 5 to 50 μm.

The above solute atom may be at least one atom selected from the groupconsisting of carbon, nitrogen and oxygen atoms.

Preferably, the hardened layer has a specular surface whose Vickershardness (HV) is 500 or greater.

The above deformed layer is generally formed in a surface of stainlesssteel of a base material for a timepiece exterior part produced byforging capable of realizing a high degree of deformation.

The above hardening is generally carried out at a temperature which isclose to the recrystallization temperature of the stainless steel orbelow. The hardening can be performed at temperature over therecrystallization temperature of the stainless steel. However, undersuch temperature conditions, the orange peel would not occur to make itunnecessary to form the deformed layer.

Austenitic stainless steel is preferably employed as the stainless steelfor use in the present invention.

The terminology “exterior part of timepiece” used herein means, forexample, a wristwatch band, bezel, casing, back lid, buckle and dial.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of the structure of band pieces produced inExample A1 of the present invention.

FIG. 2 is a schematic view of the structure of band pieces produced inExample A2 of the present invention.

FIG. 3 is a schematic view of the structure of band pieces produced inExample A3 of the present invention.

FIG. 4 is a schematic view of the structure of band pieces produced inExample A3 of the present invention.

FIG. 5 is a schematic view showing a surface treatment for band piecescarried out in Example A4 of the present invention.

FIG. 6 is a schematic view showing a further surface treatment for ofband pieces carried out in Example A4 of the present invention.

FIG. 7 is a schematic view of the structure of band pieces produced inExample A5 of the present invention.

FIG. 8 is a schematic view of the structure of band pieces produced inExample A5 of the present invention.

FIG. 9 is a schematic view showing a surface treatment for band piecescarried out in Example A6 of the present invention.

FIG. 10 is a schematic view of the structure of band pieces produced inExample A6 of the present invention.

FIG. 11 is a schematic view showing a surface treatment for band piecescarried out in Example A7 of the present invention.

FIG. 12 is a schematic view showing a further surface treatment for bandpieces carried out in Example A7 of the present invention.

FIG. 13 is a schematic view of the structure of band pieces produced inExample A7 of the present invention.

FIG. 14 is a schematic view showing a surface treatment for band piecescarried out in Example A8 of the present invention.

FIG. 15 is a schematic view showing a further surface treatment for bandpieces carried out in Example A8 of the present invention.

FIG. 16 is a schematic view of the structure of band pieces produced inExample A8 of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The decorative item of the present invention and the process forproducing the same will first be described below.

The decorative item of the present invention comprises a base materialhaving a hardened layer wherein a solute atom is diffused so as to forma solid solution; and at least one hard coating disposed on a surface ofthe hardened layer. Optionally, the decorative item may further comprisea gold alloy coating disposed on a surface of the hard coating.

Base Material

The base material for use in the decorative item of the presentinvention can be, for example, any of stainless steel, titanium metalsand titanium alloys.

In particular, austenitic stainless steel is preferably employed as thestainless steel.

The titanium metals refer to metallic materials based mainly on puretitanium, and include, for example, titanium type 1, titanium type 2 andtitanium type 3 specified in the Japanese Industrial Standards (JIS).

The titanium alloys refer to metallic materials comprising metals basedmainly on pure titanium and, added thereto, aluminum, vanadium, iron orthe like. The titanium alloys include, for example, titanium type 60 andtitanium type 60E specified in the Japanese Industrial Standards (JIS).Further, the titanium alloys include various other titanium alloys andvarious titanium-based intermetallic compounds.

In the present invention, a solute atom is diffused in a surface of theabove base material constituted of a metal or alloy so as to form asolid solution, thereby providing a hardened layer.

The above solute atom may be at least one atom selected from the groupconsisting of carbon, nitrogen and oxygen atoms.

Carbon atoms may be diffused in stainless steel, for example, austeniticstainless steel. Alternatively, nitrogen atoms together with oxygenatoms may be diffused in stainless steel.

Nitrogen atoms together with oxygen atoms may be diffused in titanium-ora titanium alloy. Alternatively, carbon atoms may be diffused intitanium or a titanium alloy.

The hardened layer is preferably so formed as to extend from a surfaceof the base material to a depth of 5 to 50 μm.

Preferably, the hardened layer has a specular surface whose Vickershardness (HV; loaded with 50 g) is 500 or greater.

In the present invention, for example, the formation of a carburizedlayer as the hardened layer in the base material constituted ofaustenitic stainless steel containing no titanium metals is preferablycarried out through the following process.

(1) Fluorination:

Before the formation of a carburized layer, it is preferred that thebase material be fluorinated in a fluorogas atmosphere at 100 to 500°C., especially 150 to 300° C.

The above austenitic stainless steel can be, for example, Fe—Cr—Ni—Mostainless steel or Fe—Cr—Mn stainless steel. Stable stainless steelwhose Ni content is minimized is preferably employed as the austeniticstainless steel in the present invention from the viewpoint of depth ofcarburized hardened layer and cost. From the viewpoint of corrosionresistance, however, stainless steel whose Ni content is high andcontaining a valence element Mo in an amount of about 1.5 to 4% byweight is preferably employed. As the optimum austenitic stainlesssteel, there can be mentioned stainless steel obtained by adding 1.5 to4% by weight of Mo to stable stainless steel whose chromium content isin the range of 15 to 25% by weight and wherein the austenitic phase isstable despite working effected at ordinary temperatures.

The fluorogas for use in the above fluorination can be, for example, agas of any of fluorocompounds such as NF₃, CF₄, SF₄, C₂F₆, BF₃, CHF₃,HF, SF₆, WF₆. SiF₄ and CIF₃. These fluorocompound gases can be usedindividually or in combination. Also, besides these gases, gases ofother compounds containing fluorine in molecules thereof can be used asthe above fluorogas. Further, F₂ gas formed by thermal cracking of thesefluorocompound gases by means of a thermal cracking apparatus or F₂ gasotherwise prepared in advance can be used as the above fluorogas. Theabove fluorocompound gases and F₂ gas can be used in an arbitrarycombination.

The fluorogases such as the above fluorocompound gases and F₂ gas,although they can be used alone, are generally diluted with an inert gassuch as nitrogen gas or argon gas before use. The concentration offluorogas per se in the diluted gas is generally in the range of 10,000to 100,000 ppm by volume, preferably 20,000 to 70,000 ppm by volume, andstill preferably 30,000 to 50,000 ppm by volume.

The fluorogas most favorably employed in the present invention is NF₃.The NF₃ is gaseous at ordinary temperatures and has high chemicalstability, and its handling is easy. The NF₃ gas is generally combinedwith nitrogen gas and used at concentrations which fall within the aboverange.

The fluorination of the present invention is carried out by disposing,for example, a base material wrought into a given shape in a fluorogasatmosphere of the above concentration at 100 to 500° C. The period offluorination, although varied depending on the type and size offluorinated material, etc., is generally in the range of ten-odd minutesto some hours.

This fluorination leads to formation of a fluorinated coating highlypermeable for carbon atoms on the surface of the base material.Accordingly, the subsequent gas carburization as hardening operationcauses carbon atoms to penetrate and diffuse from the surface ofstainless steel to the internal part thereof, so that a carburizedhardened layer can be formed easily.

(2) Gas Carburization:

The thus fluorinated base material is gas carburized in a carburizinggas atmosphere containing carbon monoxide at 400 to 500° C., preferably400 to 480° C.

The carburizing gas for use in this carburization contains carbonmonoxide as a carbon source gas. It is generally used in the form of amixed gas composed of carbon monoxide, hydrogen, carbon dioxide andnitrogen.

In the present invention, by virtue of the gas carburization at lowtemperatures ranging from 400 to 500° C., crystalline chromium carbidesuch as Cr₂₃C₆ would not precipitate in the carburized hardened layer toavoid consumption of chromium atoms of the austenitic stainless steel.As a result, the carburized hardened layer can maintain excellentcorrosion resistance. Further, by virtue of the low carburizationtemperature, bulking of crystalline chromium carbide such as Cr₂₃C₆,Cr₇C₃ or Cr₃C₂ by the carburization would not occur, and strengthlowering due to softening of the internal part of stainless steel wouldbe slight.

As a result of the above gas carburization, the carburized hardenedlayer (carbon diffusion penetration layer) is uniformly formed at thesurface of the basis material constituted of austenitic stainless steel.Furthermore, the above gas carburization would not lead to occurrence ofcrystalline chromium carbide and to consumption of chromium atoms of thebasis material (also referred to as “base material”). As a result, thecarburized hardened layer can maintain corrosion resistance that issubstantially equal to the excellent corrosion resistance inherentlypossessed by the austenitic stainless steel.

A layer wherein mainly C and Fe of stainless steel are simultaneouslypresent, probably “mill scale” containing iron oxides such as Fe₂O₃, isformed on the surface of basis material after the gas carburization.

(3) Pickling:

After the above gas carburization, the basis material for the decorativeitem, for example, the base material for the exterior part of thetimepiece is pickled. For example, the base material for the exteriorpart of the timepiece is immersed in an acid solution.

The acid solution for use in this pickling is not particularly limited.For example, it can be a solution of any of hydrofluoric acid, nitricacid, hydrochloric acid, sulfuric acid and ammonium fluoride. Theseacids can be used alone, and also can be used in the form of a solutionof a mixture of ammonium fluoride and nitric acid, a mixture of nitricacid and hydrofluoric acid, a mixture of nitric acid and hydrochloricacid or a mixture of sulfuric acid and nitric acid.

Although the concentration of acid solution can be appropriatelydetermined, with respect to, for example, a solution of a mixture ofnitric acid and hydrochloric acid, it is preferred that the nitric acidconcentration range from about 15 to 40% by weight and that thehydrochloric acid concentration range from about 5 to 20% by weight.With respect to a nitric acid solution, it is preferred that theconcentration thereof range from about 10 to 30% by weight.

The acid solution can be used at both ordinary temperatures and hightemperatures.

For the pickling, electrolysis may be performed with the use of anelectrolytic solution such as nitric acid or sulfuric acid.

The immersion time in the acid solution, although depending on the typeof acid solution, is generally in the range of about 15 to 90 min.

By virtue of this pickling, the iron contained in the mill scale formedon the surface of the base material for exterior part of timepiece,attributed to the carburization, is oxidized and diffused away to attainremoval of the mill scale. However, only this pickling cannot accomplishcomplete removal of the mill scale. Further, the surface of hardenedlayer formed by the gas carburization is roughened by the dissolution ofiron caused by the immersion in the acid solution.

(4) Rinsing:

After the above pickling, the basis material for the decorative item,for example, the base material for the exterior part of the timepiece isrinsed (washed).

By virtue of this rinsing, not only is any mill scale being peeled fromthe base material for the exterior part of the timepiece washed away butalso the acid solution attaching to the base material for the exteriorpart of the timepiece is completely washed away so as to stop theadvance of the roughening of the carburized hardened layer by the acidsolution. Despite the above pickling and rinsing, the mill scale cannotbe completely removed from the surface of the base material for theexterior part of the timepiece.

(5) Polishing:

After the rinsing, the surface of the basis material for the decorativeitem, for example, the base material for the exterior part of thetimepiece is subjected to barrel polishing.

For example, the base material for the exterior part of the timepiece isset inside a barrel vessel of a barrel polishing machine. Preferably,walnut chips and alumina abrasive as polishing mediums are placed in thebarrel vessel. Barrel polishing is carried out for a period of about 10hr to polish the rough surface formed at the outermost surface of thecarburized hardened layer as well as the remaining mill scale.

The mill scale formed on the surface of the base material for theexterior part the timepiece can be completely removed by the combinationof the above pickling, rinsing and barrel polishing. Even if the basematerial for the exterior part of the timepiece has a complexconfiguration, the mill scale can be completely removed therefrom.Further, the base material for the exterior part of the timepiece can bepolished by the barrel polishing so as to have a specular surface.

When buffing is carried out in place of the barrel polishing, it isextremely difficult to completely remove the mill scale formed on thesurface of the base material for the exterior part of the timepiece.

If the surface hardness (HV) of the carburized layer after the barrelpolishing is at least 500 as measured under a load of 50 g, it issatisfactory as that of the exterior part of the timepiece and otherdecorative items. It is preferred that the surface hardness (HV) be atleast 600 as measured under a load of 50 g.

In the present invention, after the barrel polishing, the surface of thebasis material for the decorative item such as the base material for theexterior part of the timepiece may further be buffed.

After the buffing, if the surface hardness (HV) of the carburized layeris at least 500 as measured under a load of 50 g, it is satisfactory asthat of the exterior part of the timepiece and other decorative items.It is preferred that the surface hardness (HV) be at least 600 asmeasured under a load of 50 g.

Hard Coating

It is preferred that the hard coating as a constituent of the decorativeitem of the present invention be constituted of a nitride, carbide,oxide, nitrido-carbide or nitrido-carbido-oxide of an element belongingto Group 4a, 5a or 6a of the periodic table. In particular, a hardcoating of carbon is especially preferred.

An intermediate layer can be disposed between this hard coating ofcarbon and a surface of the hardened layer of the basis material.

It is preferred that the intermediate layer comprise a lower layer of Tior Cr disposed on the hardened layer surface of the basis material andan upper layer of Si or Ge disposed on a surface of the lower layer.

In the decorative item of the present invention, at least two hardcoatings may be formed on the hardened layer surface of the basismaterial, or at least two hard coatings may be laminated on the hardenedlayer surface of the basis material.

Further, in the decorative item of the present invention, the hardcoating may be disposed on a portion of the hardened layer surface ofthe basis material.

The hard coating as a constituent of the decorative item of the presentinvention may be one exhibiting a tone which is different from that ofthe surface of the basis material.

The surface hardness of the hard coating is generally greater than thatof the basis material.

Specific methods of forming the above hard coating and specific methodsof forming the intermediate layer disposed between the hard coating ofcarbon and the surface of the hardened layer of basis material will bedescribed later with reference to Example A's.

Gold Alloy Coating

The decorative item of the present invention may further comprise a goldalloy coating disposed on the hard coating.

It is preferred that the gold alloy coating be constituted of, forexample, an alloy of gold and at least one metal selected from the groupconsisting of Al, Si, V, Cr, Ti, Fe, Co, Ni, Cu, Zn, Ge, Y, Zr, Nb, Mo,Ru, Rh, Pd, Ag, Cd, In, Sn, Hf, Ta, W, Ir and Pt.

Specific methods of forming the above gold alloy coating will bedescribed later with reference to Example A's.

Now, the exterior part of the timepiece according to the presentinvention and the process for producing the same will be described indetail.

The exterior part of a timepiece according to the present invention canbe classified into a wristwatch band as obtained by connecting aplurality of band pieces of stainless steel to each other by means of aplurality of connecting parts of stainless steel and an exterior part ofa timepiece other than the wristwatch band.

With respect to the band pieces and connecting parts constituting theformer wristwatch band, at least the band pieces are carburized,preferably gas carburized, so that a carburized hardened layer is formedat the surface thereof.

The latter exterior part of the timepiece other than the wristwatch bandis also carburized, preferably gas carburized, so that a carburizedhardened layer is formed at the surface thereof.

In the production of the wristwatch band comprising a plurality of bandpieces connected to each other by means of a plurality of connectingparts, not only the band pieces but also the connecting parts(connecting pins, length adjustment pins, etc.) are carburized, so thata hard carburized layer is formed in the connecting parts in a regionextending from the surface thereof to a depth of tens of microns (μm).As a result, the hardness of the connecting parts is increased, so that,even if the band is stretched along the length thereof, the connectingparts, such as connecting pins and length adjustment pins, are resistantto bend or breakage. Therefore, even if extremely large external forceis applied to the wristwatch band, accidental disconnection of bandpieces is rare. Thus, the strength of the band comprising a large numberof band pieces connected to each other is high.

Upon the formation of the carburized hardened layer, it may occur thatthe elastic force of length adjustment pins is changed, and hence thatdetaching of the length adjustment pins becomes difficult, or contrarilyeasy. In such an instance, after the barrel polishing step and furtherbuffing step, it is preferred to replace the carburized lengthadjustment pins by noncarburized length adjustment pins.

In the above expression “with respect to the band pieces and connectingparts constituting the former wristwatch band, at least the band piecesare carburized, preferably gas carburized, so that a carburized hardenedlayer is formed at the surface thereof”, the terminology “at least theband pieces” means that, after the barrel polishing step and furtherbuffing step, the carburized length adjustment pins may be replaced bynoncarburized length adjustment pins.

In the wristwatch band comprising a plurality of band pieces ofstainless steel connected to each other by means of a plurality ofconnecting parts of stainless steel according to the present invention,not only the band pieces but also the connecting parts are carburized,preferably gas carburized, before or after the connection of a pluralityof band pieces by means of a plurality of connecting parts.

It is especially preferred that the stainless steel for use as thematerial of the wristwatch band or constituent parts thereof (bandpieces or connecting parts) be austenitic stainless steel. The stainlesssteel for use in the present invention does not contain titanium metals.

The above austenitic stainless steel is a stainless steel whose at least60% by weight has an austenite phase at ordinary temperatures. Forexample, there can be mentioned an Fe—Cr—Ni—Mo stainless steel or anFe—Cr—Mn stainless steel. As the austenitic stainless steel for use inthe present inventions, while a stable stainless steel whose Ni contentis minimized is preferred from the viewpoint of depth of carburizedhardened layer and cost, a stainless steel having a high Ni content andcontaining Mo being a valence element in an amount of about 1.5 to 4% byweight is preferred from the viewpoint of corrosion resistance. As themost suitable austenitic stainless steel, there can be mentioned oneobtained by adding 1.5 to 4% by weight of Mo to a stable stainless steelhaving a chromium content of 15 to 25% by weight wherein the austenitephase is stable even if processed at ordinary temperatures.

Machining

In the present invention, the surface of the base materials for bandpieces connected to each other by means of connecting parts, or bandpieces prior to connection, or personal ornaments, can be machined priorto the fluorination in order to obtain exterior parts of timepiece withsurfaces having been subjected to machining, such as hairline finishingwherein a vast plurality of mutually parallel nicks are engraved orhoning wherein a vast plurality of recessed portions are cut.

With respect to the surface of the base materials for band piecesconnected to each other by means of connecting parts, or band piecesprior to connection, or personal ornaments, the carburized hardenedlayer formed in the surface by gas carburizing is so hard that machiningthereof is extremely difficult. The machining is performed prior to thefluorination because of working convenience.

The depth of hairline, honing recessed portions or the like, engraved bythe above machining in the surface of the base materials for band piecesor exterior parts of timepiece other than the wristwatch band, isnaturally to be such that hairline or honing patterns appear even afterbarrel polishing and further buffing described later. In the machining,the depth of hairline, honing recessed portions or the like, althoughnot particularly limited, is generally in the range of about 5 to 7 μm.After the barrel polishing and further buffing, the depth of hairline,honing recessed portions or the like is generally in the range of about1 to 2 μm.

Moreover, in the present invention, the above machining can be performedon the surface having been polished by the barrel polishing and furtherbuffing described later so as to be specular. With respect to thecarburized layer, the concentration of carbon of solid solution islowered in accordance with the increase of the depth from the surfacethereof, so that the hardness of the layer is decreased. Therefore,removing the region to about 1-2 μm from the extremely hard surface ofcarburized hardened layer by the barrel polishing and further buffingleads to slight lowering of the surface hardness of the carburizedhardened layer. The resultant polished surface can be machined.

This machining is not performed when the surface of the base materialsfor band pieces connected to each other by means of connecting parts, orband pieces prior to connection or exterior parts of timepiece otherthan the wristwatch band, is rendered specular.

For causing both the specular surface and the machined surface to besimultaneously present, conventional methods can be employed. Forexample, first masking portions to be rendered specular, subsequentlymachining and thereafter removing the mask enables machining only thenonmasked portions while the masked portions are specular.

With respect to the surface hardness (HV) of the carburized layer havingundergone the above machining, 500 or greater under a load of 50 g issatisfactory as the hardness of the exterior parts of the timepiece. Itis preferred that the surface hardness be 600 or greater under a load of50 g.

Fluorination

In the wristwatch band comprising a plurality of band pieces ofstainless steel connected to each other by means of a plurality ofconnecting parts of stainless steel according to the present invention,not only the band pieces but also the connecting parts are fluorinatedby heating them in a fluorogas atmosphere at 250 to 600° C., preferably300 to 500° C., before or after the connection of a plurality of bandpieces of stainless steel by means of a plurality of connecting parts ofstainless steel.

Also, in the exterior parts of the timepiece other than the wristwatchband comprising band pieces connected to each other by means ofconnecting pails, the base material thereof (base material for exteriorparts of timepiece) is fluorinated by heating it in a fluorogasatmosphere at 250 to 600° C., preferably 300 to 500° C.

Fluorogases are employed in the fluorination.

Examples of fluorogases employed in this fluorination, examples ofpreferred fluorogases, the fluorogas concentration in the use thereofand the method of application thereof are the same as describedhereinbefore with respect to the decorative item of the presentinvention and the process for producing the same.

In the present invention, this fluorination is carried out, for example,by placing, after shaping into given morphology, stainless steel bandpieces and connecting parts for the wristwatch band, or wristwatchbezels, casings, back lids, dials, etc., in a fluorination furnace andheating them in a fluorogas atmosphere of the aforementionedconcentration at 250 to 600° C. The fluorination time, althoughdepending on the type and size of materials to be fluorinated, isgenerally in the range of ten-odd-minutes to tens of minutes.

As a result of this fluorination, the passive coating containing-Cr₂O₃formed at the surface of materials to be fluorinated is converted to afluorinated coating. This fluorinated coating exhibits highpenetrability for carbon atoms. Therefore, in the subsequent gascarburizing, carbon atoms are penetrated and diffused from the surfaceof stainless steel toward the internal part thereof, thereby enablingeasily forming the carburized hardened layer.

Gas Carburizing

The thus fluorinated base materials for band pieces, connecting parts orother exterior parts of wristwatch are gas carburized at 400 to 500° C.,preferably 400 to 480° C., in a carburizing gas atmosphere containingcarbon monoxide.

In the carburizing gas for use in this carburization, carbon monoxide isused as a carbon source gas. The carburizing gas is generally applied inthe form of a mixed gas consisting of carbon monoxide and any ofhydrogen, carbon dioxide and nitrogen.

The carburization capability (carbon potential: Pc value) of thecarburizing gas is generally expressed by the formula:Pc=(Pco)² /Pco ₂

-   -   wherein Pco and Pco₂ represent the partial pressure of CO and        partial pressure of CO₂ in the gas atmosphere, respectively.

In accordance with the increase of this Pc value, the carburizationcapability is enhanced, and the surface carbon concentration ofstainless steel, for example, austenitic stainless steel is increased toincrease the surface hardness, but the amount of soot formed in the gascarburization furnace is also increased. However, even if the Pc valueis set over a given limiting point, there is a limit in the surfacehardness of resultant carburized hardened layer. On the other hand, inaccordance with the decrease of the Pc value, the carburizationcapability is diminished and the surface carbon concentration ofaustenitic stainless steel is lowered to result in lowering of thesurface hardness.

In the present invention, by lowering the gas carburization temperatureto 400 to 500° C., precipitation of crystalline chromium carbide such asCr₂₃C₆ in the carburized hardened layer and hence consumption ofchromium atoms in the austenitic stainless steel can be avoided toenable maintaining the excellent corrosion resistance of the carburizedhardened layer. Further, because of the low carburization temperature,not only can bulky enlargement of chromium carbides by the carburizationbe avoided but also the strength lowering by softening of the internalpart of stainless steel is slight.

By virtue of this gas carburization, the carburized hardened layer(layer wherein carbon is diffused and penetrated) is homogeneouslyformed at the surface of the base materials for austenitic stainlesssteel band pieces and connecting parts thereof, or other austeniticstainless steel exterior parts of timepiece.

None of crystalline chromium carbides such as Cr₂₃C₆, Cr₇C₃ and Cr₃C₂,is formed in the above carburized hardened layer, and only ultrafinemetal carbides with a particle diameter of 0.1 μm or less are recognizedby an observation through a transmission electron microscope. As aresult of spectral analysis with the use of a transmission electronmicroscope, it is found that the ultrafine metal carbides have the samechemical composition as that of the base material and are notcrystalline chromium carbides. In the carburized hardened layer, carbonatoms are penetrated and diffused in the metal lattice of the basematerial and do not form chromium carbides. The carburized hardenedlayer consists of the same austenitic phase as that of the basematerial. Because of the penetration and dissolution of a large amountof carbon atoms, the carburized hardened layer suffers a conspicuouslattice strain. By virtue of a combined effect attained by the ultrafinemetal carbides and the lattice strain, a hardness enhancement of thecarburized hardened layer is realized. Thus, a Vickers hardness (HV) ashigh as 700 to 1050 can be attained. Furthermore, because the above gascarburizing does not lead to formation of crystalline chromium carbidesand to consumption of chromium atoms in the base material, thecarburized hardened layer has the same level of excellent corrosionresistance as that inherently possessed by the austenitic stainlesssteel.

An extremely thin mill scale is formed on the gas carburized surface ofthe base materials for band pieces and connecting parts thereof, orother exterior parts of timepiece.

Pickling

The base materials for band pieces and connecting parts thereof, orother exterior parts of the timepiece, after the above gas carburizing,are pickled in the same manner as described hereinbefore with respect tothe decorative item of the present invention and the process forproducing the same. For example, the base materials for band pieces andconnecting parts thereof, or other exterior parts of the timepiece areimmersed in an acid solution.

Iron contained in the mill scale, which has been formed on the surfaceof the base materials for band pieces and connecting parts thereof, orother exterior parts of timepiece as a result of the carburizing, isoxidized and diffused in the acid solution by the pickling. Thus, themill scale is removed. However, the mill scale cannot be completelyremoved by the pickling only. Moreover, the surface of band pieces,etc., namely the surface of the carburized hardened layer formed by thegas carburizing, is roughened because of the dissolution of iron causedby the immersion in the acid solution.

Rinsing

After the above pickling, the base materials for band pieces andconnecting parts thereof, or other exterior parts of timepiece, arerinsed.

This rinsing enables not only washing away any mill scale being peeledfrom the base materials for band pieces and connecting parts thereof, orother exterior parts of timepiece, but also completely washing away theacid solution sticking to the base materials for band pieces andconnecting parts thereof, or other exterior parts of timepiece, so as toavoid further advance of roughening of the carburized hardened layer bythe acid solution. However, the mill scale formed on the surface of thebase materials for band pieces and connecting parts thereof, or otherexterior parts of timepiece, cannot be completely removed by the abovepickling and rinsing.

Barrel Polishing

The rinsed surface of the base materials for band pieces and connectingparts thereof, or other exterior parts of timepiece, is subjected tobarrel polishing.

For example, the base materials for the wristwatch band obtained byconnecting band pieces to each other by means of connecting parts, orband pieces and connecting parts prior to connection, or other exteriorparts of timepiece, are disposed inside a barrel vessel of a barrelpolishing apparatus. Polishing mediums, preferably walnut chips andalumina abrasive, are placed in the barrel vessel. A barrel polishing isperformed over a period of about 10 hr, thereby removing rough facesformed on the outermost surface of the carburized hardened layer of bandpieces, etc. and also the mill scale remaining on the outermost surface.

The mill scale formed on the surface of the base materials for mutuallyconnected band pieces, unconnected band pieces, connecting parts to beemployed for connecting band pieces to each other, or other exteriorparts of timepiece, can be completely removed by sequentially carryingout the pickling, the rinsing and the barrel polishing. Even if the basematerials for these exterior parts of timepiece have complexconfiguration, the mill scale can be completely removed. When themachining such as hairline finishing is not effected, the surface of thebase materials for mutually connected band pieces, unconnected bandpieces, connecting parts to be employed for connecting band pieces toeach other, or other exterior parts of timepiece, can be renderedspecular by this barrel polishing.

In the event that buffing is performed in place of the barrel polishing,it is very difficult to completely remove the mill scale formed on thesurface of the base materials for mutually connected band pieces,unconnected band pieces, connecting parts to be employed for connectingband pieces to each other, or other exterior parts of timepiece.

With respect to the surface hardness (HV) of the carburized layer havingundergone the above barrel polishing, 500 or greater under a load of 50g is satisfactory as the hardness of exterior parts of timepiece. It ispreferred that the surface hardness be 600 or greater under a load of 50g.

Buffing

After the barrel polishing, the surface of the base materials for bandpieces, mutually connected band pieces, or other exterior parts oftimepiece, may further be buffed.

With respect to the surface hardness (HV) of the carburized layer havingundergone the above buffing, 500 or greater under a load of 50 g issatisfactory as the hardness of exterior parts of timepiece. It ispreferred that the surface hardness be 600 or greater under a load of 50g.

Connection of a Plurality of Band Pieces

Unconnected band pieces are connected to each other by means ofconnecting parts to thereby obtain a completed wristwatch band.

Another form of exterior part of timepiece according to the presentinvention and the process for producing the same will be described indetail below.

First, another form of exterior part of timepiece according to thepresent invention will be described.

Another Form of Exterior Part of Timepiece

Another form of exterior part of timepiece according to the presentinvention comprises a metal, this metal having at its surface a deformedlayer containing a fibrous structure wherein metal crystal grains aredeformed so as to be fibrous, at least the deformed layer having ahardened layer wherein a solute atom is diffused so as to form a solidsolution.

The metal for constituting this exterior part of timepiece can be, forexample, any of stainless steel, titanium metals and titanium alloys. Inparticular, stainless steel, especially austenitic stainless steel, ispreferably employed as the above metal.

The above deformed layer is a layer provided at a metal surface andcontaining a fibrous structure wherein metal crystal grains are deformedso as to be fibrous. For forming the fibrous structure wherein metalcrystal grains are deformed so as to be fibrous, it is needed to apply aphysical external force to at least surface of the metal. It ispreferred that the deformed layer be formed by application to the metalsurface of a physical external force capable of drawing the metalsurface substantially unidirectionally.

As means for applying the physical external force to the metal surface,there can be mentioned polishing or grinding.

As the polishing, there can be mentioned, for example, customary buffingor burnishing.

In the present invention, the metal surface can be sequentiallyburnished and buffed. Also, prior to the buffing or burnishing of themetal surface, the metal surface can be subjected to barrel polishing.Further, grinding or cutting can be performed on the metal surface priorto the buffing or burnishing of the metal surface.

It is preferred that the deformed layer extend from the metal surface toa depth of 2 to 100 μm.

In the present invention, a hardened layer wherein a solute atom isdiffused so as to form a solid solution is formed at the surface of theabove deformed layer, so that, after the formation of the hardened layeras well, the metal crystal grains are fibrous. As a result, no heightdifference occurs between crystal grains and crystal grain boundaries todisenable viewing any orange peels by the naked eye. Therefore, exteriorparts of timepiece having a smooth or specular surface can be obtained.This smooth or specular surface may be planar, or curved.

It is preferred that the hardened layer extend from a surface of thedeformed layer to a depth of 5 to 50 μm.

The solute atom is at least one atom selected from the group consistingof carbon, nitrogen and oxygen atoms.

The specular surface of the hardened layer preferably exhibits a Vickershardness (HV) of 500 or greater.

Process for Producing Another Form of Exterior Part of Timepiece

Formation of Deformed Layer

In the above other form of exterior part of timepiece according to thepresent invention, a physical external force is applied to a surface ofstainless steel so as for at least the stainless steel surface to have adeformed layer containing a fibrous structure wherein metal crystalgrains are deformed so as to be fibrous.

This deformed layer is preferably formed by application to the stainlesssteel surface of a physical external force capable of drawing thestainless steel surface substantially unidirectionally.

As means for applying the physical external force to the metal surface,there can be mentioned polishing or grinding.

As the polishing, there can be mentioned, for example, customary buffingor burnishing.

In this burnishing, the base material for an exterior part of timepieceis fixed on the circumferential surface of a rotary wheel so that theupper surface of the base material is arranged outwards. Subsequently,the rotary wheel is rotated, and diamond or an ultrahard tool (forexample, tungsten or carbide) is pressed to the upper surface of thebase material to polish the upper surface of the base material.

In the present invention, the metal surface can be sequentiallyburnished and buffed. Also, prior to the buffing or burnishing of themetal surface, the metal surface can be subjected to barrel polishing.Further, grinding or cutting can be performed on the metal surface priorto the buffing or burnishing of the metal surface.

In this grinding, the upper surface of the base material for an exteriorpart of timepiece is pressed to a grinding wheel (emery wheel) beingrotated so that the upper surface of the base material for an exteriorpart of timepiece is ground by abrasive grains of the grinding wheel. Inthe present invention, grinding is performed with a grinding powermoderated. The method of moderating the grinding power, for example,comprises using less coarse abrasive grains in the grinding wheel, orreducing the number of such abrasive grains, or reducing the amount ofgrinding agent.

In the present invention, the deformed layer can be formed by subjectingthe stainless steel surface to at least one of cutting and grindingoperations to form a face of desired shape, and polishing the face ofdesired shape to form the deformed layer. Alternatively, not only a faceof desired shape but also the deformed layer can be formed by grindingthe stainless steel surface.

The face of desired shape may be substantially flat, or curved.

With respect to the individual means for applying a physical externalforce, the order of preference is:

-   -   burnishing>buffing>grinding>cutting.

In the present invention, it is especially preferred to employ grindingand buffing in combination.

It is preferred that the deformed layer be so formed as to extend fromthe stainless steel surface to a depth of 2 to 100 μm.

In the present invention, the above deformed layer is generally formedat the surface of stainless steel as the base material for an exteriorpart of timepiece, produced by forging (cold forging or hot forging)ensuring a large extent of deformation.

Formation of Hardened Layer

Now, a hardened layer is formed by subjecting the surface of the thusformed deformed layer to such a hardening that a solute atom is diffusedin the surface of the deformed layer so as to form a solid solutiontherein.

At least one atom selected from the group consisting of carbon, nitrogenand oxygen atoms is used as the above solute atom. For example, carbonatoms are diffused in the austenitic stainless steel, or nitrogen andoxygen atoms are diffused in titanium or a titanium alloy.Alternatively, carbon atoms are diffused in titanium or a titaniumalloy.

The hardened layer is preferably so formed as to extend from the surfaceof the deformed layer to a depth of 5 to 50 μm.

Preferably, the hardened layer has a specular surface whose Vickershardness (HV) is 500 or greater.

In the present invention, for example, when a carburized layer as thehardened layer is formed on the surface of the deformed layer obtainedin the above manner in the basis material for an exterior part of thetimepiece, constituted of austenitic stainless steel containing notitanium metals, an exterior part of the timepiece is preferablyproduced through the following process.

Specifically, in this process, it is preferred that, before theformation of a carburized layer, the basis material for an exterior partof timepiece having the deformed layer provided at the surface thereofbe fluorinated in a fluorogas atmosphere at 100 to 500° C., especially150 to 300° C.

The above austenitic stainless steel can be, for example, Fe—Cr—Ni—Mostainless steel or Fe—Cr—Mn stainless steel. Stable stainless steelwhose Ni content is minimized is preferably employed as the austeniticstainless steel in the present invention from the viewpoint of depth ofcarburized hardened layer and cost. From the viewpoint of corrosionresistance, however, stainless steel whose Ni content is high andcontaining a valence element Mo in an amount of about 1.5 to 4% byweight is preferably employed. As the optimum austenitic stainlesssteel, there can be mentioned stainless steel obtained by adding 1.5 to4% by weight of Mo to stable stainless steel whose chromium content isin the range of 15 to 25% by weight and wherein the austenitic phase isstable despite working effected at ordinary temperatures.

Examples of fluorogases employed in this fluorination, examples ofpreferred fluorogases, the fluorogas concentration in the use thereofand the method of application thereof are the same as describedhereinbefore with respect to the decorative item of the presentinvention and the process for producing the same.

In the present invention, this fluorination is carried out, for example,by placing, after shaping into given morphology, stainless steel bandpieces for the wristwatch band, or wristwatch bezels, casings, backlids, dials, etc., in a fluorination furnace and heating them in afluorogas atmosphere of the aforementioned concentration at 100 to 500°C. The fluorination time, although depending on the type and size ofmaterials to be fluorinated, is generally in the range of ten-oddminutes to some hours.

This fluorination leads to formation of a fluorinated coating highlypermeable for carbon atoms on the surface of the deformed layer.Accordingly, the subsequent gas carburization as hardening operationcauses carbon atoms to penetrate and diffuse from the surface ofstainless steel to the internal part thereof, so that a carburizedhardened layer can be formed easily.

The thus fluorinated base material for an exterior part of timepiece isgas carburized in the same manner as described above with respect to theexterior part of timepiece (including wristwatch band) of the presentinvention and the process for producing the same. That is, thefluorinated base material is gas carburized in a carburizing gasatmosphere containing carbon monoxide at 400 to 500° C., preferably 400to 480° C.

In the present invention, by virtue of the gas carburization at lowtemperatures ranging from 400 to 500° C., crystalline chromium carbidesuch as Cr₂₃C₆ would not precipitate in the carburized hardened layer toavoid consumption of chromium atoms of the austenitic stainless steel.As a result, the carburized hardened layer can maintain excellentcorrosion resistance. Further, by virtue of the low carburizationtemperature, bulking of chromium carbides by the carburization would notoccur, and strength lowering due to softening of the internal part ofstainless steel would be slight.

By virtue of this gas carburization, the carburized hardened layer(layer wherein carbon is diffused and penetrated) is homogeneouslyformed at the surface of the austenitic stainless steel base materialsfor exterior parts of timepiece.

None of crystalline chromium carbides such as Cr₂₃C₆, Cr₇C₃ and Cr₃C₂,is formed in the above carburized hardened layer, and only ultrafinemetal carbides with a particle diameter of 0.1 μm or less are recognizedby an observation through a transmission electron microscope. As aresult of spectral analysis with the use of a transmission electronmicroscope, it is found that the ultrafine metal carbides have the samechemical composition as that of the base material and are notcrystalline chromium carbides. In the carburized hardened layer, carbonatoms are penetrated and diffused in the metal lattice of the basematerial and do not form chromium carbides. The carburized hardenedlayer consists of the same austenitic phase as that of the basematerial. Because of the penetration and dissolution of a large amountof carbon atoms, the carburized hardened layer suffers a conspicuouslattice strain. By virtue of a combined effect attained by the ultrafinemetal carbides and the lattice strain, a hardness enhancement of thecarburized hardened layer is realized. Thus, a Vickers hardness (HV) ashigh as 700 to 1050 can be attained. Furthermore, because the above gascarburizing does not lead to formation of crystalline chromium carbidesand to consumption of chromium atoms in the base material, thecarburized hardened layer has the same level of excellent corrosionresistance as that inherently possessed by the austenitic stainlesssteel.

An extremely thin mill scale is formed on the gas carburized surface ofthe base materials for exterior parts of timepiece.

The thus gas carburized base materials for exterior parts of timepieceare pickled in the same manner as described hereinbefore with respect tothe decorative item of the present invention and the process forproducing the same.

Iron contained in the mill scale, which has been formed on the surfaceof the base materials for exterior parts of timepiece as a result of thecarburizing, is oxidized and diffused by this pickling. Thus, the millscale is removed. However, the mill scale cannot be completely removedby the pickling only. Moreover, the surface of the carburized hardenedlayer formed by the gas carburizing is roughened because of thedissolution of iron caused by the immersion in an acid solution.

After the above pickling, the base materials for exterior parts oftimepiece are rinsed.

This rinsing enables not only washing away any mill scale being peeledfrom the base materials for exterior parts of timepiece, but alsocompletely washing away the acid solution sticking to the base materialsfor exterior parts of timepiece, so as to avoid further advance ofroughening of the carburized hardened layer by the acid solution.However, the mill scale formed on the surface of the base materials forexterior parts of timepiece cannot be completely removed by the abovepickling and rinsing.

The surface of the base materials for exterior parts of timepiece afterthe rinsing is subjected to barrel polishing.

For example, the base materials for exterior parts of timepiece aredisposed inside a barrel vessel of a barrel polishing apparatus.Polishing mediums, preferably walnut chips and alumina abrasive, areplaced in the barrel vessel. A barrel polishing is performed over aperiod of about 10 hr, thereby removing rough faces formed on theoutermost surface of the carburized hardened layer and also the millscale remaining on the outermost surface.

The mill scale formed on the surface of the base materials for exteriorparts of timepiece can be completely removed by sequentially carryingout the pickling, the rinsing and the barrel polishing. Even if the basematerials for exterior parts of timepiece have complex configuration,the mill scale can be completely removed. Further, the surface of thebase materials for exterior parts of timepiece can be rendered specularby this barrel polishing.

In the event that buffing is performed in place of the barrel polishing,it is very difficult to completely remove the mill scale formed on thesurface of the base materials for exterior parts of timepiece.

With respect to the surface hardness (HV) of the carburized layer havingundergone the above barrel polishing, 500 or greater under a load of 50g is satisfactory as the hardness of exterior parts of timepiece. It ispreferred that the surface hardness be 600 or greater under a load of 50g.

In the present invention, after the barrel polishing, the surface of thebase materials for exterior parts of timepiece may further be buffed.

With respect to the surface hardness (HV) of the carburized layer havingundergone the above buffing, 500 or greater under a load of 50 g issatisfactory as the hardness of exterior parts of timepiece. It ispreferred that the surface hardness be 600 or greater under a load of 50g.

The decorative item of the present invention comprises a basis materialhaving a hardened layer extending from a surface thereof to an arbitrarydepth wherein a solute atom is diffused so as to form a solid solution;and at least one hard coating disposed on a surface of the hardenedlayer of the basis material. By virtue of this structure, the decorativeitem has an enhanced surface hardness and hence is excellent in scratchresistance.

Further, the decorative item of the present invention can have a goldalloy coating superimposed on an entire surface or part of surface ofthe above hard coating. Therefore, the decorative item can exhibitgolden color or other various tones without detriment to the surfacehardness to have enhanced ornamental value.

The process for producing a decorative item according to the inventionenables obtaining the above decorative item, such as an exterior part oftimepiece, according to the present invention with high productivity.

Moreover, according to the present invention, there can be provided anexterior part of timepiece (including a wristwatch band) of stainlesssteel, especially austenitic stainless steel, which is excellent inscratch resistance and has a specular surface without detriment to theexcellent corrosion resistance inherently possessed by austeniticstainless steel; an exterior part of timepiece (including a wristwatchband) of stainless steel, especially austenitic stainless steel, whichis excellent in scratch resistance and has a surface subjected tomachining, such as hairline finishing or honing, without detriment tothe excellent corrosion resistance inherently possessed by austeniticstainless steel; and processes for producing these exterior parts oftimepiece.

Another form of the exterior part of the timepiece according to thepresent invention comprises a metal as a base material therefor, themetal having at its surface a deformed layer containing a fibrousstructure wherein metal crystal grains are deformed so as to be fibrous,at least the deformed layer having a hardened layer wherein a soluteatom is diffused so as to form a solid solution. By virtue of thisstructure, the exterior part of the timepiece has a smooth or specularsurface free of “orange peel” and is thus exce3llent in appearance.

The process for producing another form of exterior part of timepieceaccording to the present invention enables providing the above otherform of exterior part of timepiece with excellent appearance accordingto the present invention.

EXAMPLE

The present invention will be further illustrated below with referenceto the following Examples, which in no way limit the scope of theinvention.

[Examples Relating to the Decorative Item of the Present Invention andthe Process for Producing the Same].

Example A1

A base material of austenitic stainless steel SUS 316 was shaped by hotforging, cold forging, cutting and drilling and the like into wristwatchband pieces.

A plurality of band pieces were rotatably connected to each other byinserting connecting parts in pinholes provided by drilling in each ofthe band pieces. The surface of the thus connected band pieces wasbuffed or otherwise polished so as to become specular. Thus, wristwatchbands were completed.

In each wristwatch band comprising a large number of band piecesconnected to each other, some of the band pieces are those wherein eachis separable from neighboring band pieces so as to enable regulating theband length in conformity with the size of the wrist of the wearer,i.e., band pieces for length regulation. The band pieces other than thelength regulation band pieces are those which are connected to eachother so that each is not easily separable from neighboring band pieces.

As the connecting parts, use was made of connecting parts for connectinglength regulation band pieces to each other (length regulation pins) andconnecting parts for connecting other band pieces to each other(connecting pins, split pipes and knurled pins).

The wristwatch bands were placed in a metallic muffle furnace andheated. The temperature was raised to 480° C., and a fluorogas (mixedgas consisting of 5% by volume of NF₂ and 95% by volume of N₂) was blowninto the muffle furnace for 15 min. Thus, the wristwatch bands werefluorinated.

The fluorogas was discharged from the muffle furnace. While blowing acarburizing gas (mixed gas consisting of 10% by volume of CO, 20% byvolume of H₂, 1% by volume of CO₂ and 69% by volume of N₂), thewristwatch bands were held in the muffle furnace at 480° C. for 12 hr,thereby carburizing the wristwatch bands. The wristwatch bands weretaken out from the muffle furnace.

Formation of mill scale was observed on the surface of the wristwatchbands having been carburized and taken out.

The wristwatch bands were immersed in an acid aqueous solutioncontaining 3 to 5% by volume of ammonium fluoride and 2 to 3% by volumeof nitric acid for 20 min.

As a result of this pickling, iron contained in the mill scale formed onthe surface of band pieces was oxidized and diffused, so that most ofthe mill scale was removed. Further, no mill scale was observed oninterfaces of mutually neighboring band pieces, pinhole inside walls,and connecting parts for connecting band pieces to each other, inparticular, connecting pins, split pipes and length regulation pins.

However, the surface of band pieces, namely the surface of thecarburized layer formed by the carburization, was roughened by thedissolution of iron caused by the immersion in the acid aqueoussolution.

The pickled wristwatch bands were rinsed.

The rinsed wristwatch bands were disposed inside a barrel vessel of abarrel polishing apparatus. Walnut chips and alumina abrasive aspolishing mediums were placed in the barrel vessel. A barrel polishingwas performed over a period of about 10 hr, thereby removing rough facesformed on the outermost surface of the carburized layer of band pieces.As a result of this barrel polishing, a region of the carburized layerextending from the surface thereof to a depth of 1 to 2 μm was removed,and the surface of band pieces, namely the outermost surface of thecarburized layer, became specular.

The wristwatch bands with a specular surface, obtained by the aboveprocessing, was excellent in scratch resistance and maintained the sameexcellent corrosion resistance as inherently possessed by SUS 316. Thesurface hardness (HV) of the carburized layer reached 700 under a loadof 50 g.

In the above process, because a large number of band pieces werecollected and formed into each wristwatch band before being subjected tothe fluorination, gas carburizing, pickling, rinsing and barrelpolishing, handling of the band pieces was easy to realize a highproductivity. Labor and time consumed in the processing were reduced,thereby enabling lowering processing cost.

The connecting parts were also carburized so that a hard carburizedlayer was formed in a region of each of the connecting parts extendingfrom the surface thereof to a depth of tens of microns (μm). As aresult, the hardness of the connecting parts was increased, so thatbending or breakage of connecting pins and length regulation pins wasseldom, even when the wristwatch band was stretched along the lengththereof.

However, the connecting parts such as connecting pins and lengthregulation pins remain held in the pinholes provided in band pieces bydrilling, so that it is difficult to remove the mill scale formed on theconnecting parts by pickling and polishing. In the event that mill scaleremains on the connecting parts after the pickling and polishing, it isdesirable to replace the connecting parts with mill scale remainingthereon by new connecting parts. This replacement leads to formation ofwristwatch bands whose connecting parts only are not furnished withcarburized layers.

Among the connecting parts, when mill scale remains on length regulationpins, pulling the length regulation pins out of the band pieces becomesdifficult, so that it becomes difficult to regulate the band length inconformity with the size of the wrist of the wearer. In that event, itis desirable to replace only the length regulation connecting parts,among the connecting parts, by new connecting parts. This replacementleads to formation of wristwatch bands whose length regulationconnecting parts only are not furnished with carburized layers.

A golden hard coating was formed on the carburized layer of each of theband pieces.

Referring to FIG. 1, TiN coating 3 constituted of titanium nitride, as agolden hard coating, was formed by the ion plating technique being a dryplating technique on carburized layer 2 formed at a surface of bandpiece 1.

The method of forming the TiN coating 3 will be described below.

Wristwatch band furnished with carburized layer 2 was rinsed with anorganic solvent such as isopropyl alcohol and disposed in an ion platingapparatus. Common ion plating apparatus can be employed, so that adescription and drawing with respect to the ion plating apparatus willbe omitted herein.

The ion plating apparatus was exhausted to 1.0×10⁻⁵ Torr, and thereafteran argon gas as an inert gas was introduced therein up to 3.0×10⁻³ Torr.

A thermionic filament and a plasma electrode disposed inside the ionplating apparatus were operated to produce argon plasma. Simultaneously,a voltage of −50 V was applied to the wristwatch band, and a bombardmentcleaning of the wristwatch band was performed for 10 min.

The introduction of argon gas was discontinued, and a nitrogen gas wasintroduced in the ion plating apparatus up to 2.0×10⁻³ Torr.

Subsequently, plasma was produced by means of an electron gun disposedinside the ion plating apparatus. In the plasma, titanium was evaporatedfor 10 min, so that 0.5 μm thick TiN coating 3 was formed on an entiresurface of the wristwatch band, i.e., the carburized layer 2 of the bandpieces 1.

Because the TiN coating 3 had the same optical characteristics as gold,the thus obtained wristwatch bands exhibited homogeneous golden tone.This enabled enhancing the ornamental value of the wristwatch bands.

The surface hardness (HV) of the band pieces 1 covered with the TiNcoating 3 reached 800 under a load of 50 g. The band pieces 1 coveredwith the TiN coating 3 had excellent resistances to wear, corrosion andscratch.

The scratch resistance of the band pieces 1 having undergone surfacehardening (carburization) was increased by the above formation of theTiN coating 3 which was harder than the carburized layer 2.

The dry plating method is not limited to the above ion platingtechnique, and use can be made of common means such as the sputteringtechnique or vacuum evaporation coating (vacuum deposition) technique.

The golden hard coating formed by the dry plating method can beconstituted of a nitride, carbide, oxide, nitrido-carbide ornitrido-carbido-oxide of any of the elements of Groups 4a, 5a and 6a ofthe periodic table (Ti, Zr, Hf, V, Nb, Ta, Cr, Mo and W).

When M represents an element of Group 4a, 5a or 6a of the periodic tableand a nitride of M is represented by MNx, the smaller than 1 the valueof x representing the degree of nitriding, the closer to light yellowthe golden tone of the coating of the nitride MNx. On the other hand,the greater than 1 the value of x representing the degree of nitriding,the more reddish the golden color of the coating. When the value of xrepresenting the degree of nitriding is in the range of 0.9 to 1.1, agolden color close to the tone of gold or a gold alloy can be realizedby the coating of the nitride MNx. In particular, when the value of xrepresenting the degree of nitriding satisfies x=1, the coating of thenitride MNx is not only a hard coating with satisfactory hardness butalso exhibits a golden color most close to the tone of gold or a goldalloy.

With respect to the carbide, oxide, nitrido-carbide ornitrido-carbido-oxide of element M of Group 4a, 5a or 6a of the periodictable as well, the coating thereof can be endowed with a golden colormost close to the tone of gold or a gold alloy by controlling the degreeof carbonization, oxidation or nitriding so as to fall within a givenrange.

The TiN coating and a ZrN coating are especially preferred because theseare not only hard coatings with satisfactory hardness but also exhibit agolden color most close to the tone of gold or a gold alloy.

When the thickness of the coating of the nitride MNx is extremely small,the coating cannot have effective resistances to wear, corrosion andscratch. On the other hand, when the thickness is extremely large, thetime required for coating formation is prolonged to unfavorably increasecoating cost. Accordingly, the thickness of the coating of the nitrideMNx is preferably controlled so as to fall within the range of 0.1 to 10μm, still preferably 0.2 to 5 μm.

Example A2

A hard coating with a tone different from that of Example A1 was formedon the band pieces furnished with carburized layers in the same manneras in Example A1.

Referring to FIG. 2, TiC coating 4 constituted of titanium carbide, as ahard coating with white tone, was formed by a dry plating technique oncarburized layer 2 formed at a surface of band piece 1.

According to the ion plating technique being a dry plating technique,titanium was evaporated in an ethylene gas atmosphere, and TiC coating 4was formed on a surface of band piece 1. Other coating conditions werethe same as in Example A1.

The thus obtained wristwatch bands, by virtue of the formation of TiCcoating 4, exhibited homogeneous white tone. This enabled enhancing theornamental value of the wristwatch bands.

The surface hardness (HV) of the band pieces 1 covered with the TiCcoating 4 reached 800 under a load of 50 g. The band pieces. 1 coveredwith the TiC coating 4 had excellent resistances to wear, corrosion andscratch.

The scratch resistance of the band pieces 1 having undergone surfacehardening (carburization) was increased by the above formation of theTiC coating 4 which was harder than the carburized layer 2.

Example A3

A carbon hard coating as a hard coating with black tone was formed onthe band pieces furnished with carburized layers in the same manner asin Example A1. The carbon hard coating, because of having excellentcharacteristics similar to those of diamond, is widely known asdiamondlike carbon (DLC).

Referring to FIG. 3, black carbon hard coating 5 was formed by a dryplating technique on carburized layer 2 formed at a surface of bandpiece 1.

The method of forming the carbon hard coating 5 was, for example, asfollows.

First, wristwatch band furnished with carburized layer 2 was rinsed withan organic solvent such as isopropyl alcohol and disposed in a vacuumapparatus. According to the radio-frequency plasma CVD technique, 2 μmthick carbon hard coating 5 was formed on the carburized layer 2 underthe following conditions:

[Coating Conditions]

-   -   gas species: methane gas,    -   coating pressure: 0.1 Torr,    -   high-frequency power: 300 watt, and    -   coating rate: 0.1 μm per minute.

As a result, the carbon hard coating 5 was formed on the carburizedlayer 2 with high adherence therebetween.

The thus obtained wristwatch bands, by virtue of the formation of carbonhard coating 5, exhibited homogeneous black tone. This enabled enhancingthe ornamental value of the wristwatch bands.

The surface hardness (HV) of the band pieces 1 covered with the carbonhard coating 5 reached 3000 to 5000. The scratch resistance of the bandpieces 1 having undergone surface hardening (carburization) wasincreased by the above formation of the carbon hard coating 5 which washarder than the carburized layer 2.

The thickness of the carbon hard coating 5 was preferably controlled soas to fall within the range of 0.0.1 to 5 μm, still preferably 0.5 to 3μm.

The formation of carbon hard coating 5 can be accomplished by, besidesthe above RFP-CVD technique, various gas-phase coating methods such asthe DC plasma CVD technique and the ECR technique. Alternatively,physical evaporation coating methods such as the ion beam technique, thesputtering technique and the ion plating technique may be employed.

Moreover, referring to FIG. 4, it is preferred to provide intermediatelayer coating 6 between the carburized layer 2 and the carbon hardcoating 5, because the adherence of the carbon hard coating 5 to thesurface of the band piece 1 is increased.

The method of forming the intermediate layer coating 6 was, for example,as follows.

Ti coating 6 a with a thickness of 0.1 μm was formed on the carburizedlayer 2 by a dry plating method, for example, the sputtering technique.Further, Si coating 6 b with a thickness of 0.3 μm was formed on the Ticoating 6 a by the sputtering technique.

Thereafter the carbon hard coating 5 with a thickness of 2 μm was formedon the Si coating 6 b by, for example, the radio-frequency plasma CVDtechnique under the aforementioned conditions.

The above Ti coating 6 a can be replaced by a chromium (Cr) coating. Theabove Si coating 6 b can be replaced by a germanium (Ge) coating.

The intermediate layer (coating) may be constituted of a single layer ofa carbide of Group IVa or Va metal, in place of the above laminatecoating. In particular, a coating of a titanium carbide containingexcess carbon is preferred from the viewpoint of a high adherencestrength to the carbon hard coating.

Example A4

A hard coating with golden tone was formed on part of a surface of eachof the band pieces furnished with carburized layers in the same manneras in Example A1.

Referring to FIG. 7, TiN coating 7 constituted of titanium nitride, as ahard coating with golden tone, was formed by the ion plating techniquebeing a dry plating technique on part of a surface of band piece 1.

The method of forming the golden TiN coating 7 on a part of surface willbe described below.

First, referring to FIG. 5, desired part of each of respective surfacesof band pieces 1 furnished with carburized layers 2 was printed with anorganic masking agent, or masking ink, of epoxy resin to form maskinglayer 8.

Subsequently, the band pieces 1 having the masking layer 8 formedthereon were rinsed with an organic solvent such as isopropyl alcoholand disposed in an ion plating apparatus.

Common ion plating apparatus can be employed, so that a description anddrawing with respect to the ion plating apparatus will be omittedherein.

The ion plating apparatus was exhausted to 1.0×10⁻⁵ Torr, and thereafteran argon gas as an inert gas was introduced therein up to 3.0×10⁻³ Torr.Then, a thermionic filament and a plasma electrode disposed inside theion plating apparatus were operated to produce argon plasma.Simultaneously, a voltage of −50 V was applied to each of the bandpieces 1, and a bombardment cleaning thereof was performed for 10 min.

The introduction of argon gas was discontinued, and a nitrogen gas wasintroduced in the ion plating apparatus up to 2.0×10⁻³ Torr.Subsequently, plasma was produced by means of a plasma gun disposedinside the ion plating apparatus. In the plasma, titanium was evaporatedfor 10 min, so that, referring to FIG. 6, TiN coating 7 was formed onthe surface of each of respective hardened layers 2 of band pieces 1 andTiN coating 7 a on the surface of the masking layer 8, both with a totalcoating thickness of 0.5 μm.

The masking layer 8 was swelled with the use of ethyl methyl ketone(EMK) or a stripping solution obtained by adding formic acid andhydrogen peroxide to ethyl methyl ketone (EMK), and the masking layer 8and the TiN coating 7 a superimposed thereon were stripped off by theliftoff method.

Thus, there were obtained band pieces having parts covered with the TiNcoating 7 to exhibit golden tone and parts not covered with any TiNcoating to exhibit silvery white of stainless steel, as shown in FIG. 7.This enabled enhancing the ornamental value of the wristwatch bands.

As masking means, use can be made of mechanical masking means in placeof the providing of chemical masking layer described in this Example.That is, masking can be accomplished by covering arbitrary parts of bandpieces with metallic caps before the formation of titanium nitride (TiN)coating and removing the metallic caps after the formation of titaniumnitride coating. When this masking means is employed, no titaniumnitride coating is formed on parts of band pieces covered with themetallic caps, while a titanium nitride coating is formed on parts ofband pieces not covered with the metallic caps.

In this Example, although the titanium nitride coating was employed asthe hard coating formed on parts of surfaces of band pieces 1, thegolden hard coating formed by the dry plating method can also beconstituted of a nitride, carbide, oxide, nitrido-carbide ornitrido-carbido-oxide of any of the elements of Groups 4a, 5a and 6a ofthe periodic table, as mentioned in Example A1.

In particular, when the titanium carbide coating employed in Example A2is formed on parts of surfaces of band pieces 1, there can be obtainedband pieces having parts covered with the titanium carbide coating toexhibit white tone and parts not covered with any titanium carbidecoating to exhibit silvery white of stainless steel.

Alternatively, when the carbon hard coating employed in Example A3 isformed on parts of surfaces of band pieces 1, there can be obtained bandpieces having parts covered with the carbon hard coating to exhibitblack tone and parts not covered with any carbon hard coating to exhibitsilvery white of stainless steel.

Example A5

A hard coating with golden tone was formed on the surface of band piecesfurnished with carburized layers in the same manner as in Example A1.Further, a gold alloy coating was formed on the golden hard coating.

Referring to FIG. 8, TiN coating 9 constituted of titanium nitride, as ahard coating with golden tone, was formed by the ion plating techniquebeing a dry plating technique on the surface of band piece 1 furnishedwith carburized layer 2. Gold-titanium alloy coating 10 as a gold alloycoating was formed on the TiN coating 9.

The method of forming the TiN coating 9 and gold-titanium alloy coating10 of this Example will be described below.

First, bands furnished with carburized layers 2 were rinsed with anorganic solvent such as isopropyl alcohol and disposed in an ion platingapparatus. Common ion plating apparatus can be employed, so that adescription and drawing with respect to the ion plating apparatus willbe omitted herein.

The ion plating apparatus was exhausted to 1.0×10⁻⁵ Torr, and thereafteran argon gas as an inert gas was introduced therein up to 3.0×10⁻³ Torr.

Then, a thermionic filament and a plasma electrode disposed inside theion plating apparatus were operated to produce argon plasma.Simultaneously, a voltage of −50 V was applied to each of the bandpieces 1, and a bombardment cleaning thereof was performed for 10 min.

Subsequently, plasma was produced by means of a plasma gun disposedinside the ion plating apparatus. In the plasma, titanium was evaporatedfor 10 min, so that 0.5 μm thick TiN coating 9 was formed on the entiresurface of band pieces 1.

Thereafter, the evaporation of titanium and the introduction of argongas were discontinued, and the ion plating apparatus was exhausted to1.0×10⁻⁵ Torr. An argon gas was introduced in the ion plating apparatusup to 1.0×10⁻³ Torr, and plasma was produced. In the plasma, agold-titanium mixture consisting of 50 atomic % of gold and 50 atomic %of titanium was evaporated, thereby forming gold-titanium alloy coating10. When the thickness of gold-titanium alloy coating 10 became 0.3 μm,the evaporation of the gold-titanium mixture was terminated.

The thus obtained band pieces exhibited homogeneous golden tone. Thisenabled enhancing the ornamental value of the wristwatch bands. Further,the formation of gold-titanium alloy coating 10 as an outermost layercoating enabled obtaining wristwatch bands which exhibited golden tonehaving greater warmth than that of the TiN coating 9. This enabledlending enhanced beauty to the wristwatch bands.

Generally, the gold alloy coating per se cannot have effectiveresistances to wear, corrosion and scratch unless the thickness thereofis greater than 10 μm. Gold is a very expensive metal. Therefore,increasing the thickness of the gold alloy coating invites an extremeincrease of coating cost. However, in this Example, the hard TiN coatingwas provided under the outermost layer coating constituted of a goldalloy. This TiN coating has excellent resistances to wear, corrosion andscratch, so that the outermost layer coating constituted of a gold alloycan be thinned. Accordingly, in this Example, there is such an advantagethat the usage of expensive gold can be reduced by the sequentialformation of TiN coating and thin gold alloy coating, thereby enablinglowering coating cost.

Although the thin formed outermost layer coating constituted of a goldalloy might be partially worn to expose the underlying TiN coating, anypartial wearing of the outermost layer coating would never beconspicuous. The reason is that the TiN coating has the same opticalcharacteristics as gold and exhibits golden tone. Even if the outermostlayer coating constituted of a gold alloy with golden tone is partiallyworn, the underlying TiN coating with the same golden tone is exposedthere. Therefore, even if the outermost layer coating constituted of agold alloy is thinned, the wearing is not visible to enable maintainingthe beauty of wristwatch bands as a personal ornament and the ornamentalvalue thereof.

In this Example, although the titanium nitride coating was employed asthe hard coating, the golden hard coating formed by the dry platingmethod can also be constituted of a nitride, carbide, oxide,nitrido-carbide or nitrido-carbido-oxide of any of the elements ofGroups 4a, 5a and 6a of the periodic table.

The gold alloy coating can be, besides the above gold-titanium alloycoating, a coating constituted of an alloy of gold and at least onemetal selected from the group consisting of Al, Si, V, Cr, Fe, Co, Ni,Cu, Zn, Ge, Y, Zr, Nb, Mo, Ru, Rh, Pd, Ag, Cd, In, Sn, Hf, Ta, W, Ir andPt.

However, when personal ornaments covered with coatings of some goldalloys selected from among the above combinations are brought intocontact with the skin, metal ions may be leached by an electrolyticsolution such as sweat and have a possibility for causing metal allergywhen contacted with human skin equipped with the personal ornaments. Inparticular, leached nickel ions are known as the metal ion for which thegreatest number of metal allergy cases are reported. In contrast, ironis the metal for which the number of metal allergy cases is extremelysmall. No metal allergy case has been reported with respect to titanium.Therefore, from the viewpoint of metal allergy, it is preferred that thegold alloy coating as the outermost layer coating be constituted of agold-iron alloy or a gold-titanium alloy.

Example A6

The gold alloy coating described in Example A5 may further be formedonly on the hard coating with golden tone partially formed on thesurface of band pieces furnished with carburized layers as described inExample A4.

The method of partially forming TiN coating 11 constituted of titaniumnitride as a hard coating with golden tone and gold-titanium alloycoating 12 as a gold alloy coating will be briefly described withreference to FIGS. 9 and 10.

First, referring to FIG. 9, desired part of each of respective surfacesof band pieces 1 furnished with carburized layers 2 was printed with anorganic masking agent, or masking ink, of epoxy resin to form maskinglayer 8.

Subsequently, the band pieces 1 having the masking layer 8 formedthereon were rinsed with an organic solvent such as isopropyl alcoholand disposed in an ion plating apparatus.

According to the ion plating technique being a dry plating technique,TiN coating 11, 11 a was formed on the surface of carburized layers 2 ofband pieces 1 and the surface of the masking layer 8 so that the totalcoating thickness became 0.5 μm. Thereafter, 0.3 μm thick gold-titaniumalloy coating 12, 12 a was formed on the TiN coating 11, 11 a.

The masking layer 8 was swelled with the use of ethyl methyl ketone(EMK) or a stripping solution obtained by adding formic acid andhydrogen peroxide to ethyl methyl ketone (EMK), and the masking layer 8and the TiN coating 11 a and gold-titanium alloy coating 12 asuperimposed thereon were stripped off by the liftoff method.

Thus, there were obtained wristwatch bands having parts covered with theTiN coating 11 and gold-titanium alloy coating 12 to exhibit golden toneand parts not covered with such coatings to exhibit silvery white ofstainless steel, as shown in FIG. 10.

In this Example, as mentioned in Example A5, use can be made of varioushard coatings other than the titanium nitride coating. Also, use can bemade of various gold alloy coatings other than the gold-titanium alloycoating.

Example A7

The first hard coating was formed on the surface of band piecesfurnished with carburized layers in the same manner as in Example A1.Further, the second hard coating with tone different from that of thefirst hard coating was formed on part of the surface of the first hardcoating.

Referring to FIG. 11, golden tone TiN coating 3 constituted of titaniumnitride as the first hard coating was formed on the surface of bandpieces 1 furnished with carburized layers 2 in the same manner as inExample A1. Masking layer 13 was formed on desired part of the surfaceof the TiN coating 3 by, for example, printing with an organic maskingagent, or masking ink, of epoxy resin.

Subsequently, referring to FIG. 12, white tone TiC coating 14constituted of titanium carbide as the second hard coating was formed onthe surface of the coating 3, and the TiC coating 14 a on the surface ofthe masking layer 13, in the same manner as in Example A2.

Thereafter, the masking layer 13 was swelled with the use of a strippingsolution, and the masking layer 13 and the TiC coating 14 a superimposedthereon were stripped off by the liftoff method.

Thus, there were obtained band pieces having parts exhibiting white toneof the TiC coating 14 superimposed on the surface of golden TiN coating3 and parts exhibiting golden tone of the TiN coating 3, as shown inFIG. 13. This enabled enhancing the ornamental value of the wristwatchbands. Further, the scratch resistance of the surface-hardened(carburized) band pieces 1 was increased by the superimposition of theTiN coating 3 and TiC coating 14 harder than the carburized layers 2.

In this Example, as in Example A5, use can be made of various hardcoatings other than the titanium nitride and titanium carbide coatings.Also, either of the first hard coating and the second hard coating canbe replaced by the carbon hard coating of Example A3. The types ofmasking layer 13 and stripping solution can appropriately be selected inconformity with the type of such coatings.

When M represents an element of Group 4a, 5a or 6a of the periodic tableand a nitride of M is represented by MNx, both the first hard coatingand the second hard coating can be constituted of MNx. If the first hardcoating and the second hard coating are constituted so that the value ofx representing the degree of nitriding of the former is different fromthat of the latter, the tone of the first hard coating can bedifferentiated from that of the second hard coating. This is true withrespect to the carbide, oxide, nitrido-carbide and nitrido-carbido-oxideas well.

Example A8

The first hard coating was formed on part of the surface of band piecesfurnished with carburized layers in the same manner as in Example A1.Further, the second hard coating with tone different from that of thefirst hard coating was formed on the other part of the surface of bandpieces.

Referring to FIG. 14, golden tone TiN coating 7 constituted of titaniumnitride as the first hard coating was formed on part of the surface ofband pieces 1 furnished with carburized layers 2 in the same manner asin Example A4. Masking layer 15 was formed on the surface of the TiNcoating 7 and desired part of the surface of band pieces 1 continuingtherefrom.

Subsequently, referring to FIG. 15, white tone TiC coating 16constituted of titanium carbide as the second hard coating was formed onthe TiN coating 7, the masking layer 15 and remaining part of thesurface of band pieces 1, in the same manner as in Example A2.

Thereafter, the masking layer 15 was swelled with the use of a strippingsolution, and the masking layer 15 and the TiC coating 16 superimposedthereon were stripped off by the liftoff method.

Thus, there were obtained triple color band pieces having parts coveredwith the TiN coating 7 to exhibit golden tone, parts covered with theTiC coating 16 to exhibit white tone and parts where the surface of bandpieces 1 is exposed, as shown in FIG. 16. This enabled enhancing theornamental value of the wristwatch bands.

Selection of the first hard coating and second hard coating andselection of the stripping solution and masking layer are the same asdescribed in Example A7. The same gold alloy coating as described inExample A5 may be formed on either or both of the first hard coating andthe second hard coating.

Although the ion plating technique was employed as the dry platingmethod in the above Examples A2 and A4 to A8, use can be made of othercommon coating forming methods such as the sputtering technique andvacuum evaporation coating technique.

In all the above Examples, the invention was described with reference tohand pieces for wristwatch bands. However, the present invention is alsoapplicable to items for accommodating mechanical or electronic drivingmechanism, such as a wristwatch casing. Still further, the presentinvention is applicable to all other decorative items (includingcomponents thereof).

[Examples Relating to the Exterior Part of Timepiece According to thePresent Invention and the Process for Producing the Same]

Example B1

A base material of austenitic stainless steel SUS 316 was shaped by hotforging, cold forging, cutting and drilling into wristwatch band pieces.

A plurality of band pieces were rotatably connected to each other byinserting connecting parts in pinholes provided by drilling in each ofthe band pieces. The surface of the thus connected band pieces wasbuffed or otherwise polished so as to become specular. Thus, wristwatchbands were completed.

In each wristwatch band comprising a large number of band piecesconnected to each other, some of the band pieces are those wherein eachis separable from neighboring band pieces so as to enable regulating theband length in conformity with the size of the wrist of the wearer,i.e., band pieces for length regulation. The band pieces other than thelength regulation band pieces are those which are connected to eachother so that each is not easily separable from neighboring band pieces.As the connecting parts, use was made of connecting parts for connectinglength regulation band pieces to each other (length regulation pins) andconnecting parts for connecting other band pieces to each other(connecting pins, split pipes and knurled pins).

The wristwatch bands were placed in a metallic muffle furnace andheated. The temperature was raised to 480° C., and a fluorogas (mixedgas consisting of 5% by volume of NF₂ and 95% by volume of N₂) was blowninto the muffle furnace for 15 min. Thus, the wristwatch bands werefluorinated.

The fluorogas was discharged from the muffle furnace. While blowing acarburizing gas (mixed gas consisting of 10% by volume of CO, 20% byvolume of H₂, 1% by volume of CO₂ and 69% by volume of N₂), thewristwatch bands were held in the muffle furnace at 480° C. for 12 hr,thereby carburizing the wristwatch bands. The wristwatch bands wereremoved from the muffle furnace.

Formation of mill scale was observed on the surface of the wristwatchbands having been carburized and taken out.

The wristwatch bands were immersed in an acid aqueous solutioncontaining 3 to 5% by volume of ammonium fluoride and 2 to 3% by volumeof nitric acid for 20 min.

As a result of this pickling, iron contained in the mill scale formed onthe surface of band pieces was oxidized and diffused, so that most ofthe mill scale was removed. Further, no mill scale was observed oninterfaces of mutually neighboring band pieces, pinhole inside walls,and connecting parts for connecting band pieces to each other, inparticular, connecting pins, split pipes and length regulation pins.

However, the surface of band pieces, namely the surface of thecarburized layer formed by the carburization, was roughened by thedissolution of iron caused by the immersion in the acid aqueoussolution.

The pickled wristwatch bands were rinsed.

The rinsed wristwatch bands were disposed inside a barrel vessel of abarrel polishing apparatus. Walnut chips and alumina abrasive aspolishing mediums were placed in the barrel vessel. A barrel polishingwas performed over a period of about 10 hr, thereby removing rough facesformed on the outermost surface of the carburized layer of band pieces.As a result of this barrel polishing, a region of the carburized layerextending from the surface thereof to a depth of 1 to 2 μm was removed,and the surface of band pieces, namely the outermost surface of thecarburized layer, became specular.

The wristwatch bands with a specular surface, obtained by the aboveprocessing, was excellent in scratch resistance and maintained the sameexcellent corrosion resistance as inherently possessed by SUS 316. Inthe above process, because a large number of band pieces were collectedand formed into each wristwatch band before being subjected to the aboveprocessing steps, labor and time consumed in the processing werereduced, thereby enabling lowering processing cost.

The connecting parts were also carburized so that a hard carburizedlayer was formed in a region of each of the connecting parts extendingfrom the surface thereof to a depth of tens of microns (μm). As aresult, the hardness of the connecting parts was increased, so thatbending or breakage of connecting pins and length regulation pins wasrare, even when the wristwatch band was stretched along the lengththereof.

In this Example B1, because a large number of band pieces were collectedand formed into each wristwatch band before being subjected to thefluorination, gas carburizing, pickling, rinsing and barrel polishing,handling of the band pieces in these processing steps was easy torealize a high productivity.

Example B2

Wristwatch bands were produced in the same manner as in Example B1,except that, before the fluorination, a multiplicity of hairlines wereformed along the band length on outer surfaces (when worn round thewrist, surfaces arranged outside) of band pieces.

The resultant wristwatch bands had hairline-finished surfaces, whichwere excellent in scratch resistance and maintained the same excellentcorrosion resistance as inherently possessed by SUS 316.

Example B3

Bezels finished so as to have a specular surface were produced in thesame manner as in Example B1, except that the wristwatch bands werereplaced by bezels for wristwatch.

The resultant bezels were excellent in scratch resistance and maintainedthe same excellent corrosion resistance as inherently possessed by SUS316.

Example B4

Casings finished so as to have a specular surface were produced in thesame manner as in Example B1, except that the wristwatch bands werereplaced by casings for wristwatch.

The resultant casings were excellent in scratch resistance andmaintained the same excellent corrosion resistance as inherentlypossessed by SUS 316.

Example B5

Back lids finished so as to have a specular surface were produced in thesame manner as in Example B1, except that the wristwatch bands werereplaced by back lids for wristwatch.

The resultant back lids were excellent in scratch resistance andmaintained the same excellent corrosion resistance as inherentlypossessed by SUS 316.

Example B6

Dials finished so as to have a specular surface were produced in thesame manner as in Example B1, except that the wristwatch bands werereplaced by dials for wristwatch.

The resultant dials were excellent in scratch resistance and maintainedthe same excellent corrosion resistance as inherently possessed by SUS316.

[Examples Relating to the Other Form of Exterior Part of TimepieceAccording to the Present Invention and the Process for Producing theSame].

Example C1

A rodlike material of austenitic stainless steel SUS 316 was provided.The rodlike material had a rounded rectangular section conforming to themorphology of wristwatch band pieces. The rodlike material was sliced atintervals conforming to band piece widths.

Pinholes for insertion of connecting pins were drilled in the obtainedslices, thereby completing band pieces for wristwatch bands.

A plurality of band pieces were rotatably connected to each other byinserting connecting pins in pinholes provided by drilling in each ofthe band pieces, thereby assembling wristwatch bands.

Upper rounded surface (when worn round the wrist, outer surface arrangedoutside) of each of the band pieces of wristwatch bands was buffed.

A section of the buffed upper surface portion of band pieces wasobserved through an electron microscope. As a result, it was recognizedthat metal crystal grains of the stainless steel surface portion weredrawn in the direction of buff rotation by the external force applied bybuffing, thereby creating a fibrous structure of metal crystal grainsdeformed in fibrous form. A deformed layer including this fibrousstructure was formed so as to extend from the upper surface of bandpieces to a depth of 3 to 7 μm.

The wristwatch bands were placed in a metallic muffle furnace andheated. The temperature was raised to 480° C., and a fluorogas (mixedgas consisting of 5% by volume of NF₂ and 95% by volume of N₂) was blowninto the muffle furnace for 15 min. Thus, the wristwatch bands werefluorinated.

The fluorogas was discharged from the muffle furnace. While blowing acarburizing gas (mixed gas consisting of 10% by volume of CO, 20% byvolume of H₂, 1% by volume of CO₂ and 69% by volume of N₂), thewristwatch bands were held in the muffle furnace at 480° C. for 12 hr,thereby carburizing the wristwatch bands. The wristwatch bands wereremoved from the muffle furnace.

Formation of mill scale was observed on the surface of the wristwatchbands having been carburized and taken out.

The wristwatch bands were immersed in an acid aqueous solutioncontaining 3 to 5% by volume of ammonium fluoride and 2 to 3% by volumeof nitric acid for 20 min.

As a result of this pickling, iron contained in the mill scale formed onthe surface of band pieces was oxidized and diffused, so that most ofthe mill scale was removed. Further, no mill scale was observed oninterfaces of mutually neighboring band pieces, pinhole inside walls,and connecting pins for connecting band pieces to each other.

However, the surface of band pieces, namely the surface of thecarburized layer formed by the carburization, was roughened by thedissolution of iron caused by the immersion in the acid aqueoussolution.

The pickled wristwatch bands were rinsed.

The rinsed wristwatch bands were disposed inside a barrel vessel of abarrel polishing apparatus. Walnut chips and alumina abrasive aspolishing mediums were placed in the barrel vessel. A barrel polishingwas performed over a period of about 10 hr, thereby removing rough facesformed on the outermost surface of the carburized layer of band pieces.

As a result of this barrel polishing, a region of the carburized layerextending from the surface thereof to a depth of 0.3 to 1 μm wasremoved, and the upper surface of band pieces became a smooth beautifulspecular surface required for exterior parts of timepiece. On the uppersurface, “orange peel” attributed to minute unevenness was not visibleat all.

Observation of a section of band pieces after the barrel polishingthrough an electron microscope showed that the carburized layer wasformed so as to extend from the upper surface of band pieces to a depthof 18 to 20 μm.

Example C2

Cylinders of austenitic stainless steel SUS 316 were provided. Thecylinders were cold forged into the configuration of wristwatch bandpieces in such a manner that an external force is applied along thecentral axis passing through the center of a circular section of thecylinders.

Pinholes for insertion of connecting pins were drilled in the thusforged members, thereby completing band pieces for wristwatch bands.

A plurality of band pieces were rotatably connected to each other byinserting connecting pins in pinholes provided by drilling in each ofthe band pieces, thereby assembling wristwatch bands.

Upper surface (when worn round the wrist, outer surface arrangedoutside) of each of the band pieces of wristwatch bands was burnishedinto a planar specular surface. Specifically, each wristwatch band wasfixed on the circumferential surface of a rotary wheel so that the uppersurface of the wristwatch band was arranged outside. The rotary wheelwas rotated, and a diamond tool mounted on the rotary wheel was pressedto the upper surface of the wristwatch band.

A section of the burnished upper surface portion of band pieces wasobserved through an electron microscope. As a result, it was recognizedthat metal crystal grains of the stainless steel surface portion weredrawn in the direction of rotary wheel rotation by the external forceapplied by burnishing, thereby creating a fibrous structure of metalcrystal grains deformed in fibrous form. A deformed layer including thisfibrous structure was formed so as to extend from the upper surface ofband pieces to a depth of 5 to 10 μm.

The wristwatch bands were placed in a metallic muffle furnace andheated. The temperature was raised to 480° C., and a fluorogas (mixedgas consisting of 5% by volume of NF₂ and 95% by volume of N₂) was blowninto the muffle furnace for 15 min. Thus, the wristwatch bands werefluorinated.

The fluorogas was discharged from the muffle furnace. While blowing acarburizing gas (mixed gas consisting of 10% by volume of CO, 20% byvolume of H₂, 1% by volume of CO₂ and 69% by volume of N₂), thewristwatch bands were held in the muffle furnace at 480° C. for 12 hr,thereby carburizing the wristwatch bands. The wristwatch bands wereremoved from the muffle furnace.

Formation of mill scale was observed on the surface of the wristwatchbands having been carburized and taken out.

The wristwatch bands were immersed in an acid aqueous solutioncontaining 3 to 5% by volume of ammonium fluoride and 2 to 3% by volumeof nitric acid for 20 min.

As a result of this pickling, iron contained in the mill scale formed onthe surface of band pieces was oxidized and diffused, so that most ofthe mill scale was removed. Further, no mill scale was observed oninterfaces of mutually neighboring band pieces, pinhole inside walls,and connecting pins for connecting band pieces to each other.

However, the surface of band pieces, namely the surface of thecarburized layer formed by the carburization, was roughened by thedissolution of iron caused by the immersion in the acid aqueoussolution.

The pickled wristwatch bands were rinsed.

The rinsed wristwatch bands were disposed inside a barrel vessel of abarrel polishing apparatus. Walnut chips and alumina abrasive aspolishing mediums were placed in the barrel vessel. A barrel polishingwas performed over a period of about 10 hr, thereby removing rough facesformed on the outermost surface of the carburized layer of band pieces.

As a result of this barrel polishing, a region of the carburized layerextending from the surface thereof to a depth of 0.5 to 1.5 μm wasremoved, and the upper surface of band pieces became a smooth beautifulspecular surface required for exterior parts of timepiece. On the uppersurface, “orange peel” attributed to minute unevenness was not visibleat all.

Observation of a section of band pieces after the barrel polishingthrough an electron microscope showed that the carburized layer wasformed so as to extend from the upper surface of band pieces to a depthof 18 to 20 μm.

Example C3

Wristwatch bands were assembled in the same manner as in Example C2.

Upper surface of each of the band pieces of wristwatch bands was planedby cutting operation and further buffed into a specular surface.

A section of the resultant upper surface portion of band pieces wasobserved through an electron microscope. As a result, it was recognizedthat, in the same manner as in Example C1, metal crystal grains of thestainless steel surface portion were drawn in the direction of buffrotation by the external force applied by buffing, thereby creating afibrous structure of metal crystal grains deformed in fibrous form. Adeformed layer including this fibrous structure was formed so as toextend from the upper surface of band pieces to a depth of 3 to 6 μm.

The wristwatch bands were placed in a metallic muffle furnace andheated. The temperature was raised to 480° C., and a fluorogas (mixedgas consisting of 5% by volume of NF₂ and 95% by volume of N₂) was blowninto the muffle furnace for 15 min. Thus, the wristwatch bands werefluorinated.

The fluorogas was discharged from the muffle furnace. While blowing acarburizing gas (mixed gas consisting of 10% by volume of CO, 20% byvolume of H₂, 1% by volume of CO₂ and 69% by volume of N₂), thewristwatch bands were held in the muffle furnace at 480° C. for 12 hr,thereby carburizing the wristwatch bands. The wristwatch bands wereremoved from the muffle furnace.

Formation of mill scale was observed on the surface of the wristwatchbands having been carburized and taken out.

The wristwatch bands were immersed in an acid aqueous solutioncontaining 3 to 5% by volume of ammonium fluoride and 2 to 3% by volumeof nitric acid for 20 min.

As a result of this pickling, iron contained in the mill scale formed onthe surface of band pieces was oxidized and diffused, so that most ofthe mill scale was removed. Further, no mill scale was observed oninterfaces of mutually neighboring band pieces, pinhole inside walls,and connecting pins for connecting band pieces to each other.

However, the surface of band pieces, namely the surface of thecarburized layer formed by the carburization, was roughened by thedissolution of iron caused by the immersion in the acid aqueoussolution.

The pickled wristwatch bands were rinsed.

The rinsed wristwatch bands were disposed inside a barrel vessel of abarrel polishing apparatus. Walnut chips and alumina abrasive aspolishing mediums were placed in the barrel vessel. A barrel polishingwas performed over a period of about 10 hr, thereby removing rough facesformed on the outermost surface of the carburized layer of band pieces.

As a result of this barrel polishing, a region of the carburized layerextending from the surface thereof to a depth of 1 to 2 μm was removed,and the upper surface of band pieces became a smooth beautiful specularsurface required for exterior parts of timepiece. On the upper surface,“orange peel” attributed to minute unevenness was not visible at all.

Observation of a section of band pieces after the barrel polishingthrough an electron microscope showed that the carburized layer wasformed so as to extend from the upper surface of band pieces to a depthof 20 to 25 μm.

Example C4

Wristwatch bands were assembled in the same manner as in Example C2.

Upper surface of each of the band pieces of wristwatch bands was planedinto a specular surface by grinding means with a reduced grinding power.Specifically, the upper surface of wristwatch band was pressed to agrinding (emery) wheel being rotated, so that the upper surface of bandpieces was ground by the abrasive grains of the grinding wheel.

The upper surface of band pieces was further buffed so as to obtain adesirable specular surface.

A section of the resultant upper surface portion of band pieces wasobserved through an electron microscope. As a result, it was recognizedthat, in the same manner as in Example C1, metal crystal grains of thestainless steel surface portion were drawn in the direction of rotationof grinding wheel and buff by the external force applied by the grindingand buffing, thereby creating a fibrous structure of metal crystalgrains deformed in fibrous form. A deformed layer including this fibrousstructure was formed so as to extend from the upper surface of bandpieces to a depth of 7 to 12 μm.

The wristwatch bands were placed in a metallic muffle furnace andheated. The temperature was raised to 480° C., and a fluorogas (mixedgas consisting of 5% by volume of NF₂ and 95% by volume of N₂) was blowninto the muffle furnace for 15 min. Thus, the wristwatch bands werefluorinated.

The fluorogas was discharged from the muffle furnace. While blowing acarburizing gas (mixed gas consisting of 10% by volume of CO, 20% byvolume of H₂, 1% by volume of CO₂ and 69% by volume of N₂), thewristwatch bands were held in the muffle furnace at 480° C. for 12 hr,thereby carburizing the wristwatch bands. The wristwatch bands wereremoved from the muffle furnace.

Formation of mill scale was observed on the surface of the wristwatchbands having been carburized and taken out.

The wristwatch bands were immersed in an acid aqueous solutioncontaining 3 to 5% by volume of ammonium fluoride and 2 to 3% by volumeof nitric acid for 20 min.

As a result of this pickling, iron contained in the mill scale formed onthe surface of band pieces was oxidized and diffused, so that most ofthe mill scale was removed. Further, no mill scale was observed oninterfaces of mutually neighboring band pieces, pinhole inside walls,and connecting pins for connecting band pieces to each other.

However, the surface of band pieces, namely the surface of thecarburized layer formed by the carburization, was roughened by thedissolution of iron caused by the immersion in the acid aqueoussolution.

The pickled wristwatch bands were rinsed.

The rinsed wristwatch bands were disposed inside a barrel vessel of abarrel polishing apparatus. Walnut chips and alumina abrasive aspolishing mediums were placed in the barrel vessel. A barrel polishingwas performed over a period of about 10 hr, thereby removing rough facesformed on the outermost surface of the carburized layer of band pieces.

As a result of this barrel polishing, a region of the carburized layerextending from the surface thereof to a depth of 1.5 to 2.5 μm wasremoved, and the upper surface of band pieces became a smooth beautifulspecular surface required for exterior parts of timepiece. On the uppersurface, “orange peel” attributed to minute unevenness was not visibleat all.

Observation of a section of band pieces after the barrel polishingthrough an electron microscope showed that the carburized layer wasformed so as to extend from the upper surface of band pieces to a depthof 15 to 20 μm.

In this Example C4, not only planing of the upper surface of band piecesinto a specular surface but also converting of metal crystal grainslying in the vicinity of band piece surface to the fibrous structure canbe accomplished by grinding means with a reduced grinding power, therebyenabling reducing the number of production process steps. Therefore, theemployment of this grinding enables the lowering of production cost.

Example C5

Wristwatch bands were assembled in the same manner as in Example C2.

Upper surface of each of the band pieces of wristwatch bands was planedinto a specular surface by grinding means with a reduced grinding power.Specifically, the upper surface of wristwatch band was pressed to agrinding (emery) wheel being rotated, so that the upper surface of bandpieces was ground by the abrasive grains of the grinding wheel.

A section of the resultant upper surface portion of band pieces wasobserved through an electron microscope. As a result, it was recognizedthat metal crystal grains of the stainless steel surface portion weredrawn in the direction of rotation of grinding wheel with a reducedgrinding power, thereby creating a fibrous structure of metal crystalgrains deformed in fibrous form. A deformed layer including this fibrousstructure was formed so as to extend from the upper surface of bandpieces to a depth of 2 to 5 μm.

The wristwatch bands were placed in a metallic muffle furnace andheated. The temperature was raised to 480° C., and a fluorogas (mixedgas consisting of 5% by volume of NF₂ and 95% by volume of N₂) was blowninto the muffle furnace for 15 min. Thus, the wristwatch bands werefluorinated.

The fluorogas was discharged from the muffle furnace. While blowing acarburizing gas (mixed gas consisting of 10% by volume of CO, 20% byvolume of H₂, 1% by volume of CO₂ and 69% by volume of N₂), thewristwatch bands were held in the muffle furnace at 480° C. for 12 hr,thereby carburizing the wristwatch bands. The wristwatch bands wereremoved from the muffle furnace.

Formation of mill scale was observed on the surface of the wristwatchbands having been carburized and taken out.

The wristwatch bands were immersed in an acid aqueous solutioncontaining 3 to 5% by volume of ammonium fluoride and 2 to 3% by volumeof nitric acid for 20 min.

As a result of this pickling, iron contained in the mill scale formed onthe surface of band pieces was oxidized and diffused, so that most ofthe mill scale was removed. Further; no mill scale was observed oninterfaces of mutually neighboring band pieces, pinhole inside walls,and connecting pins for connecting band pieces to each other.

However, the surface of band pieces, namely the surface of thecarburized layer formed by the carburization, was roughened by thedissolution of iron caused by the immersion in the acid aqueoussolution.

The pickled wristwatch bands were rinsed.

The rinsed wristwatch bands were disposed inside a barrel vessel of abarrel polishing apparatus. Walnut chips and alumina abrasive aspolishing mediums were placed in the barrel vessel. A barrel polishingwas performed over a period of about 10 hr, thereby removing rough facesformed on the outermost surface of the carburized layer of band pieces.

As a result of this barrel polishing, a region of the carburized layerextending from the surface thereof to a depth of 1 to 2 μm was removed,and the upper surface of band pieces became a smooth beautiful specularsurface required for exterior parts of timepiece. On the upper surface,“orange peel” attributed to minute unevenness was not visible at all.

Observation of a section of band pieces after the barrel polishingthrough an electron microscope showed that the carburized layer wasformed so as to extend from the upper surface of band pieces to a depthof 20 to 30 μm.

In this Example C5, not only planing of the upper surface of band piecesinto a specular surface but also converting of metal crystal grainslying in the vicinity of band piece surface to the fibrous structure canbe accomplished by grinding means with a reduced grinding power, therebyenabling reducing the number of production process steps. Therefore, theemployment of this grinding enables the lowering of production cost.

Example C6

Cylinders of austenitic stainless steel SUS 316 were provided. Thecylinders were cold forged into ring members in such a manner that anexternal force is applied along the central axis passing through thecenter of a circular section of the cylinders.

Each of the inner and outer boundary dimensions of the thus forged ringmembers were adjusted to the desired one by a cutting operation.

Upper surfaces of the resultant ring members were buffed to completebezels with a specular upper surface.

A section of the buffed bezels was observed through an electronmicroscope. As a result, it was recognized that metal crystal grains ofthe stainless steel surface portion were drawn in the direction of buffrotation by the external force applied by buffing, thereby creating afibrous structure of metal crystal grains deformed in fibrous form. Adeformed layer including this fibrous structure was formed so as toextend from the upper surface of bezels to a depth of 3 to 5 μm.

The bezels were placed in a metallic muffle furnace and heated. Thetemperature was raised to 480° C., and a fluorogas (mixed gas consistingof 5% by volume of NF₂ and 95% by volume of N₂) was blown into themuffle furnace for 15 min. Thus, the bezels were fluorinated.

The fluorogas was discharged from the muffle furnace. While blowing acarburizing gas (mixed gas consisting of 10% by volume of CO, 20% byvolume of H₂, 1% by volume of CO₂ and 69% by volume of N₂), the bezelswere held in the muffle furnace at 480° C. for 12 hr, therebycarburizing the bezels. The bezels were removed from the muffle furnace.

Formation of mill scale was observed on the surface of the bezels havingbeen carburized and taken out.

The bezels were immersed in an acid aqueous solution containing 3 to 5%by volume of ammonium fluoride and 2 to 3% by volume of nitric acid for20 min.

As a result of this pickling, iron contained in the mill scale formed onthe surface of bezels was oxidized and diffused, so that most of themill scale was removed.

However, the surface of bezels, namely the surface of the carburizedlayer formed by the carburization, was roughened by the dissolution ofiron caused by the immersion in the acid aqueous solution.

The pickled bezels were rinsed.

The rinsed bezels were disposed inside a barrel vessel of a barrelpolishing apparatus. Walnut chips and alumina abrasive as polishingmediums were placed in the barrel vessel. A barrel polishing wasperformed over a period of about 10 hr, thereby removing rough facesformed on the outermost surface of the carburized layer of bezels.

As a result of this barrel polishing, a region of the carburized layerextending from the surface thereof to a depth of 1 to 2 μm was removed,and the upper surface of bezels became a smooth beautiful specularsurface required for exterior parts of timepiece. On the upper surface,“orange peel” attributed to minute unevenness was not visible at all.

Observation of a section of bezels after the barrel polishing through anelectron microscope showed that the carburized layer was formed so as toextend from the upper surface of bezels to a depth of 20 to 23 μm.

1. A decorative item comprising: a basis material of a stainless steel,a carburized hardened layer extending from a surface of the basismaterial to an arbitrary depth wherein carbon is diffused so as to forma solid solution in which crystalline chromium carbide is not formed,and at least one hard coating disposed on a surface of the carburizedhardened layer of the basis material, wherein the hard coating has asurface hardness greater than that of the carburized layer.
 2. Thedecorative item as claimed in claim 1, wherein the hard coating and thebasis material at its surface exhibit respective tones which aredifferent from each other.
 3. The decorative item as claimed in claim 1,wherein the hard coating is constituted of a nitride, carbide, oxide,nitrido-carbide or nitrido-carbido-oxide of an element belonging toGroup 4a, 5a or 6a of the periodic table.
 4. The decorative item asclaimed in claim 1, wherein the hard coating is a hard coating ofcarbon.
 5. The decorative item as claimed in claim 4, further comprisingan intermediate layer disposed between the hard coating of carbon and asurface of the carburized hardened layer of the basis material.
 6. Thedecorative item as claimed in claim 5, wherein the intermediate layercomprises a lower layer of Ti or Cr disposed on the carburized hardenedlayer surface of the basis material and an upper layer of Si or Gedisposed on a surface of the lower layer.
 7. The decorative item asclaimed in claim 1, wherein at least two hard coatings are formed on thecarburized hardened layer surface of the basis material.
 8. Thedecorative item as claimed in claim 1, wherein at least two hardcoatings are laminated on the carburized hardened layer surface of thebasis material.
 9. The decorative item as claimed in claim 1, whereinthe hard coating is disposed on portion of the carburized hardened layersurface of the basis material.
 10. The decorative item as claimed inclaim 1, further comprising a gold alloy coating disposed on a surfaceof the hard coating.
 11. The decorative item as claimed in claim 10,wherein the gold alloy coating is constituted of an alloy of gold and atleast one metal selected from the group consisting of Al, Si, V, Cr, Ti,Fe, Co, Ni, Cu, Zn, Ge, Y, Zr, Nb, Mo, Ru, Rh, Pd, Ag, Cd, In, Sn, Hf,Ta, W, Ir and Pt.
 12. The decorative item as claimed in claim 1, whichis an exterior part of timepiece.
 13. The decorative item as claimed inclaim 1, wherein the stainless steel is an austenitic stainless steel.14. The decorative item as claimed in claim 1, wherein the carburizedhardened layer has a specular surface.
 15. The decorative item asclaimed in claim 1, wherein the hard coating is formed by dry plating.16. A process for producing a decorative item, comprising the steps of:providing a basis material of stainless steel, carburizing a surface ofthe basis material in a carburizing gas atmosphere containing carbonmonoxide at 400 to 500° C. to form a carburized hardened layer extendingfrom the surface of the basis material to an arbitrary depth whereincarbon is diffused so as to form a solid solution in which crystallinechromium carbide is not formed, and forming at least one hard coating ona surface of the carburized hardened layer of the basis material,wherein the hard coating has a surface hardness greater than that of thecarburized layer.
 17. The process as claimed in claim 16, wherein thedecorative item is an exterior part of a timepiece.
 18. An exterior partof a timepiece, comprising a stainless steel having at its surface acarburized layer wherein carbon is diffused so as to form a solidsolution in which crystalline chromium carbide is not formed, whereinthe carburized layer has a specular surface obtained by removing roughfaces formed on the outermost surface thereof and polishing, and havinga Vickers hardness (HV) of 500 or more.
 19. An exterior part of atimepiece, comprising a stainless steel having at its surface acarburized layer wherein carbon is diffused so as to form a solidsolution in which crystalline chromium carbide is not formed, whereinthe carburized layer has a machined surface.
 20. The exterior part oftimepiece as claimed in claim 19, wherein the machined surface has aVickers hardness (HV) of 500 or more.
 21. The exterior part of timepieceas claimed in claim 20, which is one produced by machining a surface ofan exterior part of timepiece and thereafter carburizing the machinedsurface.
 22. The exterior part of a timepiece as claimed in claim 19,wherein the machined surface is a hairline surface or a honing surface.23. A wristwatch band comprising a plurality of band pieces of stainlesssteel connected to each other, each of the band pieces having at itssurface a carburized layer wherein carbon is diffused so as to form asolid solution in which crystalline chromium carbide is not formed,wherein the carburized layer has a specular surface obtained by removingrough faces formed on the outermost surface thereof and polishing, andhaving a Vickers hardness (HV) of 500 or more.
 24. The wristwatch bandas claimed in claim 23, wherein the band pieces are connected to eachother by means of connecting parts of stainless steel, each of theconnecting parts having on at least a portion of its surface acarburized layer wherein carbon is diffused so as to form a solidsolution.
 25. The wristwatch band as claimed in claim 23, produced byconnecting the band pieces to each other by means of connecting parts,carburizing the band pieces and the connecting parts, and thereafterpolishing surfaces of the band pieces.
 26. The wristwatch band asclaimed in claim 25, which further comprises connecting parts having nocarburized layer.
 27. A wristwatch band comprising a plurality of bandpieces of stainless steel connected to each other, each of the bandpieces having at its surface a carburized layer wherein carbon isdiffused so as to form a solid solution in which crystalline chromiumcarbide is not formed, wherein the carburized layer has a machinedsurface.
 28. The wristwatch band as claimed in claim 27, wherein theband pieces are connected to each other by means of connecting parts ofstainless steel, each of the connecting parts having at at least aportion of its surface a carburized layer wherein carbon is diffused soas to form a solid solution.
 29. The wristwatch band as claimed in claim27, produced by connecting the band pieces to each other by means ofconnecting parts, carburizing the band pieces and the connecting parts,and thereafter polishing surfaces of the band pieces.
 30. The exteriorpart of a timepiece as claimed in claim 28, wherein the machined surfaceis a hairline surface or a honing surface.
 31. The wristwatch band asclaimed in claim 27, produced by connecting the band pieces to eachother by means of connecting parts, carburizing the band pieces and theconnecting parts, and thereafter polishing surfaces of the band pieces.32. The wristwatch band as claimed in claim 27, wherein the band piecesare connected to each other by means of connecting parts of stainlesssteel, each of the connecting parts having on at least a portion of itssurface a carburized layer wherein carbon is diffused so as to form asolid solution.
 33. A process for producing a wristwatch band,comprising the steps of: connecting a plurality of band pieces ofstainless steel to each other by means of a plurality of connectingparts of stainless steel; fluorinating the band pieces and theconnecting parts in a fluorogas atmosphere at 400 to 500° C.; gascarburizing the fluorinated band pieces and connecting parts in acarburizing gas atmosphere containing carbon monoxide at 400 to 500° C.to form a carburized hardened layer extending from the surface of thefluorinated band pieces and connecting parts to an arbitrary depthwherein carbon is diffused so as to form a solid solution in whichcrystalline chromium carbides is not formed; pickling the carburizedband pieces and connecting parts, followed by rinsing; and subjectingsurfaces of the band pieces to barrel polishing.
 34. The process asclaimed in claim 33, which further comprises buffing the band piecesurfaces having undergone barrel polishing.
 35. The process as claimedin claim 33, which further comprises machining surfaces of the bandpieces connected by means of the connecting parts prior to thefluorination to obtain a wristwatch band having machined surfaces. 36.The process as claimed in claim 34, which further comprises machiningsurfaces of the band pieces connected by means of the connecting partsprior to the fluorination to obtain a wristwatch band having machinedsurfaces.
 37. A process for producing a wristwatch band, comprising thesteps of: fluorinating a plurality of band pieces of stainless steel anda plurality of connecting parts of stainless steel in a fluorogasatmosphere at 250 to 600° C.; gas carburizing the fluorinated bandpieces and connecting parts in a carburizing gas atmosphere containingcarbon monoxide at 400 to 500° C. to form a carburized hardened layerextending from the surface of the fluorinated band pieces ad connectingparts to an arbitrary depth wherein carbon is diffused so as to form asolid solution in which crystalline chromium carbides is not formed;pickling the carburized band pieces and connecting parts, followed byrinsing; subjecting surfaces of the band pieces to barrel polishing; andconnecting the band pieces by means of the connecting parts.
 38. Theprocess as claimed in claim 37, which further comprises buffing the bandpiece surfaces having undergone barrel polishing.
 39. The process asclaimed in claim 38, which further comprises machining surfaces of theplurality of band pieces prior to the fluorination to obtain awristwatch band having machined surfaces.
 40. The process as claimed inclaim 37, which further comprises machining surfaces of the plurality ofband pieces prior to the fluorination to obtain a wristwatch band havingmachined surfaces.
 41. A process for producing an exterior part of atimepiece other than a wristwatch band, comprising the steps of:connecting a plurality of pieces of stainless steel to each other bymeans of a plurality of connecting parts of stainless steel to obtain abase material for a timepiece exterior part of stainless steel otherthan a wristwatch band; fluorinating the base material in a fluorogasatmosphere at 250 to 600° C.; gas carburizing the fluorinated basematerial in a carburizing gas atmosphere containing carbon monoxide at400 to 500° C. to form a carburized hardened layer extending from thesurface of the fluorinated base material to an arbitrary depth whereincarbon is diffused so as to form a solid solution in which crystallinechromium carbides is not formed; pickling the carburized base material,followed by rinsing; and subjecting surfaces of the base material tobarrel polishing.
 42. The process as claimed in claim 41, which furthercomprises buffing the base material surfaces having undergone barrelpolishing.
 43. The process as claimed in claim 42, which furthercomprises machining surfaces of the base material prior to thefluorination to obtain an exterior part of timepiece other thanwristwatch band having machined surfaces.
 44. The process as claimed inclaim 41, which further comprises machining surfaces of the basematerial prior to the fluorination to obtain an exterior part oftimepiece other than wristwatch band having machined surfaces.
 45. Theexterior part of a timepiece as claimed in claim 18, wherein a region ofthe carburized layer extending from the surface thereof has a depth offrom 1 to 2 μm.
 46. An exterior part of a timepiece, comprising astainless steel having at its surface a carburized layer wherein carbonis diffused so as to form a solid solution in which crystalline chromiumcarbide is not formed, wherein a machined surface formed beforecarburization is provided on the carburized layer surface.